compile_utils.py

#  Copyright 2019 The TensorFlow Authors. All Rights Reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
# ==============================================================================

"""Utilities for `Model.compile`."""


import copy

import tensorflow.compat.v2 as tf

from keras import losses as losses_mod
from keras import metrics as metrics_mod
from keras.saving import saving_lib
from keras.utils import generic_utils
from keras.utils import losses_utils
from keras.utils import tf_utils


class Container:
    """Base Container class."""

    def __init__(self, output_names=None):
        self._output_names = output_names

    def build(self, y_pred):
        if self._output_names is None:
            # In Subclass API, output names like 'output_1' are used for
            # `Metric` names.
            self._output_names = create_pseudo_output_names(y_pred)

    def _conform_to_outputs(self, outputs, struct):
        """Convenience method to conform `struct` to `outputs` structure.

        Mappings performed:

        (1) Map a dict to a list of outputs, using the output names.
        (2) Fill missing keys in a dict w/ `None`s.
        (3) Map a single item to all outputs.

        Args:
          outputs: Model predictions.
          struct: Arbitrary nested structure (e.g. of labels, sample_weights,
            losses, or metrics).

        Returns:
          Mapping of `struct` to `outputs` structure.
        """
        struct = map_to_output_names(outputs, self._output_names, struct)
        struct = map_missing_dict_keys(outputs, struct)
        # Allow passing one object that applies to all outputs.
        if not tf.nest.is_nested(struct) and tf.nest.is_nested(outputs):
            struct = tf.nest.map_structure(lambda _: struct, outputs)
        return struct

    def _maybe_broadcast_to_outputs(self, outputs, objects):
        """Determines if losses / metrics should be applied to all outputs.

        NOTE: This method should only be called for Metrics / Losses, not for
        y_true / sample_weight.

        Args:
          outputs: Model predictions.
          objects: Arbitrary nested structure (e.g. of losses or metrics)

        Returns:
          Arbitrary nested structure of objects, maybe copied to each output.

        Applies a Loss / Metric to all outputs.
        """
        if not self._should_broadcast(objects):
            return objects

        # When there is more than one Model output, this is needed to keep
        # each Metric / Loss separate. When there is only one Model output,
        # the user-supplied object should be used.
        should_copy_objects = len(tf.nest.flatten(outputs)) > 1

        def _broadcast_fn():
            if should_copy_objects:
                return tf.nest.map_structure(self._copy_object, objects)
            return objects

        return tf.nest.map_structure(lambda _: _broadcast_fn(), outputs)

    def _should_broadcast(self, objects):
        raise NotImplementedError

    def _copy_object(self, obj):
        raise NotImplementedError


class LossesContainer(Container):
    """A container class for losses passed to `Model.compile()`.

    Args:
      losses: Struct of loss function(s). See `Model.compile()` doc for more
        information.
      loss_weights: Weights of the losses contributions of different model
        outputs. See `Model.compile()` doc for more information.
      output_names: List of string. Per-output metric names.
      total_loss_mean: A `keras.metrics.Mean` instance that is used to track the
        mean of all losses (including compiled and regularization losses).
    """

    def __init__(
        self, losses, loss_weights=None, output_names=None, total_loss_mean=None
    ):
        super(LossesContainer, self).__init__(output_names=output_names)

        # Keep user-supplied values untouched for recompiling and serialization.
        self._user_losses = losses
        self._user_loss_weights = loss_weights

        self._losses = losses
        self._loss_weights = loss_weights
        self._per_output_metrics = None  # Per-output losses become metrics.

        # Mean of the total loss.
        self._total_loss_mean = total_loss_mean or metrics_mod.Mean(name="loss")
        self._built = False

    def get_config(self):
        # In case `self._losses` is a single string where we convert it to a
        # list.
        self._losses = tf.nest.flatten(self._losses)
        return {
            "losses": [
                saving_lib.serialize_keras_object(obj)
                for obj in self._losses
                if obj is not None
            ],
            "total_loss_mean": saving_lib.serialize_keras_object(
                self._total_loss_mean
            ),
        }

    @classmethod
    def from_config(cls, config):
        """Returns the `LossesContainer` instance given the `config`."""
        deserialized_config = {}
        for key, value in config.items():
            if isinstance(value, list):
                deserialized_config[key] = [
                    saving_lib.deserialize_keras_object(item) for item in value
                ]
            else:
                deserialized_config[key] = saving_lib.deserialize_keras_object(
                    value
                )
        return cls(**deserialized_config)

    @property
    def metrics(self):
        """Per-output loss metrics."""
        if not self._built:
            return []
        per_output_metrics = [
            metric_obj
            for metric_obj in tf.nest.flatten(self._per_output_metrics)
            if metric_obj is not None
        ]
        return [self._total_loss_mean] + per_output_metrics

    def build(self, y_pred):
        """One-time setup of loss objects."""
        super(LossesContainer, self).build(y_pred)

        self._losses = self._maybe_broadcast_to_outputs(y_pred, self._losses)
        self._losses = self._conform_to_outputs(y_pred, self._losses)
        self._losses = tf.nest.map_structure(
            self._get_loss_object, self._losses
        )
        self._losses = tf.nest.flatten(self._losses)

        self._loss_weights = self._maybe_broadcast_to_outputs(
            y_pred, self._loss_weights
        )
        self._loss_weights = self._conform_to_outputs(
            y_pred, self._loss_weights
        )
        self._loss_weights = tf.nest.flatten(self._loss_weights)

        self._create_metrics()
        self._built = True

    @property
    def built(self):
        return self._built

    def _create_metrics(self):
        """Creates per-output loss metrics, but only for multi-output Models."""
        if len(self._output_names) == 1:
            self._per_output_metrics = [None]
        else:
            self._per_output_metrics = []
            for loss_obj, output_name in zip(self._losses, self._output_names):
                if loss_obj is None:
                    self._per_output_metrics.append(None)
                else:
                    self._per_output_metrics.append(
                        metrics_mod.Mean(output_name + "_loss")
                    )

    def __call__(
        self, y_true, y_pred, sample_weight=None, regularization_losses=None
    ):
        """Computes the overall loss.

        Args:
          y_true: An arbitrary structure of Tensors representing the ground
            truth.
          y_pred: An arbitrary structure of Tensors representing a Model's
            outputs.
          sample_weight: An arbitrary structure of Tensors representing the
            per-sample loss weights. If one Tensor is passed, it is used for all
            losses. If multiple Tensors are passed, the structure should match
            `y_pred`.
          regularization_losses: Additional losses to be added to the total
            loss.

        Returns:
          The total loss as a `tf.Tensor`, or `None` if no loss results.
        """
        y_true = self._conform_to_outputs(y_pred, y_true)
        sample_weight = self._conform_to_outputs(y_pred, sample_weight)

        if not self._built:
            self.build(y_pred)

        y_pred = tf.nest.flatten(y_pred)
        y_true = tf.nest.flatten(y_true)
        sample_weight = tf.nest.flatten(sample_weight)

        loss_values = []  # Used for gradient calculation.
        total_loss_mean_values = []  # Used for loss metric calculation.
        batch_dim = None
        zip_args = (
            y_true,
            y_pred,
            sample_weight,
            self._losses,
            self._loss_weights,
            self._per_output_metrics,
        )
        for y_t, y_p, sw, loss_obj, loss_weight, metric_obj in zip(*zip_args):
            if (
                y_t is None or loss_obj is None
            ):  # Ok to have no loss for an output.
                continue

            y_t, y_p, sw = match_dtype_and_rank(y_t, y_p, sw)
            sw = losses_utils.apply_mask(y_p, sw, losses_utils.get_mask(y_p))
            loss_value = loss_obj(y_t, y_p, sample_weight=sw)

            total_loss_mean_value = loss_value
            # Correct for the `Mean` loss metrics counting each replica as a
            # batch.
            if loss_obj.reduction == losses_utils.ReductionV2.SUM:
                total_loss_mean_value *= (
                    tf.distribute.get_strategy().num_replicas_in_sync
                )

            if batch_dim is None:
                if tf_utils.is_ragged(y_t):
                    batch_dim = y_t.nrows()
                else:
                    batch_dim = tf.shape(y_t)[0]

            if metric_obj is not None:
                metric_obj.update_state(
                    total_loss_mean_value, sample_weight=batch_dim
                )

            if loss_weight is not None:
                loss_value *= loss_weight
                total_loss_mean_value *= loss_weight

            if (
                loss_obj.reduction
                == losses_utils.ReductionV2.SUM_OVER_BATCH_SIZE
                or loss_obj.reduction == losses_utils.ReductionV2.AUTO
            ):
                loss_value = losses_utils.scale_loss_for_distribution(
                    loss_value
                )

            loss_values.append(loss_value)
            total_loss_mean_values.append(total_loss_mean_value)

        if regularization_losses:
            regularization_losses = losses_utils.cast_losses_to_common_dtype(
                regularization_losses
            )
            reg_loss = tf.add_n(regularization_losses)
            total_loss_mean_values.append(reg_loss)
            loss_values.append(
                losses_utils.scale_loss_for_distribution(reg_loss)
            )

        if loss_values:
            total_loss_mean_values = losses_utils.cast_losses_to_common_dtype(
                total_loss_mean_values
            )
            total_total_loss_mean_value = tf.add_n(total_loss_mean_values)
            self._total_loss_mean.update_state(
                total_total_loss_mean_value, sample_weight=batch_dim
            )

            loss_values = losses_utils.cast_losses_to_common_dtype(loss_values)
            total_loss = tf.add_n(loss_values)
            return total_loss
        else:
            return None

    def reset_state(self):
        """Resets the state of loss metrics."""
        if not self._built:
            return
        metrics = [self._total_loss_mean] + tf.nest.flatten(
            self._per_output_metrics
        )
        for metric_obj in metrics:
            if metric_obj is not None:
                metric_obj.reset_state()

    def _get_loss_object(self, loss):
        """Returns a `Loss` object.

        Converts the user-supplied loss to a `Loss` object. Also allows
        `SUM_OVER_BATCH_SIZE` reduction to be used for this loss.

        Args:
          loss: A string, function, or `Loss` object.

        Returns:
          A `Loss` object.
        """
        if loss is None:
            return None  # Ok to have no loss for an output.

        loss = losses_mod.get(loss)
        if not isinstance(loss, losses_mod.Loss):
            loss_name = get_custom_object_name(loss)
            if loss_name is None:
                raise ValueError(f"Loss should be a callable, received: {loss}")
            loss = losses_mod.LossFunctionWrapper(loss, name=loss_name)
        loss._allow_sum_over_batch_size = True
        return loss

    def _should_broadcast(self, obj):
        return not tf.nest.is_nested(obj)

    def _copy_object(self, obj):
        return obj  # Losses don't need to be copied.


class MetricsContainer(Container):
    """A container class for metrics passed to `Model.compile`."""

    def __init__(
        self,
        metrics=None,
        weighted_metrics=None,
        output_names=None,
        from_serialized=False,
    ):
        """Initializes a container for metrics.

        Arguments:
          metrics: see the `metrics` argument from `tf.keras.Model.compile`.
          weighted_metrics: see the `weighted_metrics` argument from
            `tf.keras.Model.compile`.
          output_names: A list of strings of names of outputs for the model.
          from_serialized: Whether the model being compiled is from a serialized
            model.  Used to avoid redundantly applying pre-processing renaming
            steps.
        """
        super(MetricsContainer, self).__init__(output_names=output_names)

        self._check_duplicated_metrics(metrics, weighted_metrics)
        # Keep user-supplied values untouched for recompiling and serialization.
        self._user_metrics = metrics
        self._user_weighted_metrics = weighted_metrics

        self._metrics = metrics
        self._weighted_metrics = weighted_metrics
        self._built = False

        self._from_serialized = from_serialized

    def _check_duplicated_metrics(self, metrics, weighted_metrics):
        """Raise error when user provided metrics have any duplications.

        Note that metrics are stateful container, a shared metric instance
        between model.metric and model.weighted_metric will make the same
        intance to be udpated twice, and report wrong value.

        Args:
          metrics: User provided metrics list.
          weighted_metrics: User provided weighted metrics list.

        Raises:
          ValueError, when duplicated metrics instance discovered in user
            provided metrics and weighted metrics.
        """
        seen = set()
        duplicated = []
        for x in tf.nest.flatten(metrics) + tf.nest.flatten(weighted_metrics):
            # We only check metrics object. The string and function objects
            # will be converted to unique Metric instance.
            if not isinstance(x, metrics_mod.Metric):
                continue
            if x in seen:
                duplicated.append(x)
            seen.add(x)

        if duplicated:
            raise ValueError(
                "Found duplicated metrics object in the user provided "
                "metrics and weighted metrics. This will cause the same "
                "metric object to be updated multiple times, and report "
                "wrong results. \n"
                f"Duplicated items: {duplicated}"
            )

    @property
    def metrics(self):
        """All metrics in this container."""
        if not self._built:
            return []
        return self._metrics_in_order

    @property
    def unweighted_metrics(self):
        """Metrics in the container that should not be passed sample_weight."""
        if not self._built:
            return None
        return tf.nest.flatten(self._metrics)

    @property
    def weighted_metrics(self):
        """Metrics in this container that should be passed `sample_weight`."""
        if not self._built:
            return None
        return tf.nest.flatten(self._weighted_metrics)

    def build(self, y_pred, y_true):
        """One-time setup of metric objects."""
        super(MetricsContainer, self).build(y_pred)

        self._metrics = self._maybe_broadcast_to_outputs(y_pred, self._metrics)
        self._metrics = self._conform_to_outputs(y_pred, self._metrics)

        self._weighted_metrics = self._maybe_broadcast_to_outputs(
            y_pred, self._weighted_metrics
        )
        self._weighted_metrics = self._conform_to_outputs(
            y_pred, self._weighted_metrics
        )

        # Standardize on tuple since `tf.data` turns lists into `Tensor`s.
        y_pred = tf.__internal__.nest.list_to_tuple(y_pred)
        y_true = tf.__internal__.nest.list_to_tuple(y_true)
        self._metrics = tf.__internal__.nest.list_to_tuple(self._metrics)
        self._weighted_metrics = tf.__internal__.nest.list_to_tuple(
            self._weighted_metrics
        )

        # Convert to `Metric` objects, potentially disambiguating based on
        # output properties.
        self._metrics = tf.__internal__.nest.map_structure_up_to(
            y_pred, self._get_metric_objects, self._metrics, y_true, y_pred
        )
        self._weighted_metrics = tf.__internal__.nest.map_structure_up_to(
            y_pred,
            self._get_metric_objects,
            self._weighted_metrics,
            y_true,
            y_pred,
        )

        self._metrics = tf.__internal__.nest.flatten_up_to(
            y_pred, self._metrics, check_types=False
        )
        self._weighted_metrics = tf.__internal__.nest.flatten_up_to(
            y_pred, self._weighted_metrics, check_types=False
        )

        # Assumes metrics, weighted_metrics have been flattened up to outputs.
        #
        # If we are loading a model that has been already serialized, we do not
        # want to re-apply any pre-processing metric renaming steps.
        if not self._from_serialized:
            self._set_metric_names()
        self._create_ordered_metrics()
        self._built = True

    @property
    def built(self):
        return self._built

    def _set_metric_names(self):
        """Sets unique metric names."""
        # For multi-output models, prepend the output name to the metric name.
        # For weighted metrics, prepend "weighted_" if the name would be
        # non-unique.

        metric_names = set()
        is_multi_output = len(self._output_names) > 1
        zip_args = (self._output_names, self._metrics, self._weighted_metrics)
        for output_name, output_metrics, weighted_output_metrics in zip(
            *zip_args
        ):
            for m in output_metrics:
                if m is None:
                    continue
                if is_multi_output:
                    m._name = output_name + "_" + m._name
                if m._name in metric_names:
                    raise ValueError(
                        f"Found two metrics with the same name: {m._name}. "
                        "All the metrics added to the model need to have "
                        "unique names."
                    )
                metric_names.add(m._name)

            for wm in weighted_output_metrics:
                if wm is None:
                    continue
                if is_multi_output:
                    if output_name + "_" + wm._name in metric_names:
                        wm._name = output_name + "_weighted_" + wm._name
                    else:
                        wm._name = output_name + "_" + wm._name
                elif wm._name in metric_names:
                    wm._name = "weighted_" + wm._name

                if wm._name in metric_names:
                    raise ValueError(
                        "Found two weighted metrics with the same name: "
                        f"{wm._name}.All the metrics added to the model need "
                        "to have unique names."
                    )
                metric_names.add(wm._name)

    def _create_ordered_metrics(self):
        """Cache the flat order needed when return metrics, for backcompat."""
        self._metrics_in_order = []
        for output_metrics, output_weighted_metrics in zip(
            self._metrics, self._weighted_metrics
        ):
            for m in tf.nest.flatten(output_metrics):
                if m is not None:
                    self._metrics_in_order.append(m)
            for wm in tf.nest.flatten(output_weighted_metrics):
                if wm is not None:
                    self._metrics_in_order.append(wm)

    def update_state(self, y_true, y_pred, sample_weight=None):
        """Updates the state of per-output metrics."""
        y_true = self._conform_to_outputs(y_pred, y_true)
        sample_weight = self._conform_to_outputs(y_pred, sample_weight)

        if not self._built:
            self.build(y_pred, y_true)

        y_pred = tf.nest.flatten(y_pred)
        y_true = tf.nest.flatten(y_true) if y_true is not None else []
        sample_weight = tf.nest.flatten(sample_weight)

        zip_args = (
            y_true,
            y_pred,
            sample_weight,
            self._metrics,
            self._weighted_metrics,
        )
        for y_t, y_p, sw, metric_objs, weighted_metric_objs in zip(*zip_args):
            # Ok to have no metrics for an output.
            if y_t is None or (
                all(m is None for m in metric_objs)
                and all(wm is None for wm in weighted_metric_objs)
            ):
                continue

            y_t, y_p, sw = match_dtype_and_rank(y_t, y_p, sw)
            mask = losses_utils.get_mask(y_p)
            sw = losses_utils.apply_mask(y_p, sw, mask)

            for metric_obj in metric_objs:
                if metric_obj is None:
                    continue
                metric_obj.update_state(y_t, y_p, sample_weight=mask)

            for weighted_metric_obj in weighted_metric_objs:
                if weighted_metric_obj is None:
                    continue
                weighted_metric_obj.update_state(y_t, y_p, sample_weight=sw)

    def reset_state(self):
        """Resets the state of all `Metric`s in this container."""
        if self._built:
            metrics = self._metrics_in_order
        else:
            # If the user supplied `Metric` objects directly, we should
            # reset those. This could also contain `str`s or `function`s
            # though.
            metrics = tf.nest.flatten(self._user_metrics) + tf.nest.flatten(
                self._user_weighted_metrics
            )

        for metric_obj in metrics:
            if isinstance(metric_obj, metrics_mod.Metric):
                metric_obj.reset_state()

    def _get_metric_objects(self, metrics, y_t, y_p):
        """Convert user-supplied metrics to `Metric` objects."""
        metrics = tf.nest.flatten(metrics)
        return [self._get_metric_object(m, y_t, y_p) for m in metrics]

    def _get_metric_object(self, metric, y_t, y_p):
        """Converts user-supplied metric to a `Metric` object.

        Args:
          metric: A string, function, or `Metric` object.
          y_t: Sample of label.
          y_p: Sample of output.

        Returns:
          A `Metric` object.
        """
        if metric is None:
            return None  # Ok to have no metric for an output.

        # Convenience feature for selecting b/t binary, categorical,
        # and sparse categorical.
        if str(metric).lower() not in ["accuracy", "acc", "crossentropy", "ce"]:
            metric_obj = metrics_mod.get(metric)
        else:
            y_t_rank = len(y_t.shape.as_list())
            y_p_rank = len(y_p.shape.as_list())
            y_t_last_dim = y_t.shape.as_list()[-1]
            y_p_last_dim = y_p.shape.as_list()[-1]

            is_binary = y_p_last_dim == 1
            is_sparse_categorical = (
                y_t_rank < y_p_rank or y_t_last_dim == 1 and y_p_last_dim > 1
            )

            if str(metric).lower() in ["accuracy", "acc"]:
                if is_binary:
                    metric_obj = metrics_mod.binary_accuracy
                elif is_sparse_categorical:
                    metric_obj = metrics_mod.sparse_categorical_accuracy
                else:
                    metric_obj = metrics_mod.categorical_accuracy
            else:
                if is_binary:
                    metric_obj = metrics_mod.binary_crossentropy
                elif is_sparse_categorical:
                    metric_obj = metrics_mod.sparse_categorical_crossentropy
                else:
                    metric_obj = metrics_mod.categorical_crossentropy

        if isinstance(metric_obj, losses_mod.Loss):
            metric_obj._allow_sum_over_batch_size = True

        if not isinstance(metric_obj, metrics_mod.Metric):
            if isinstance(metric, str):
                metric_name = metric
            else:
                metric_name = get_custom_object_name(metric)
                if metric_name is None:
                    raise ValueError(
                        f"Metric should be a callable, received: {metric}"
                    )

            metric_obj = metrics_mod.MeanMetricWrapper(
                metric_obj, name=metric_name
            )

        return metric_obj

    def _should_broadcast(self, obj):
        # e.g. 'mse'.
        if not tf.nest.is_nested(obj):
            return True
        # e.g. ['mse'] or ['mse', 'mae'].
        return isinstance(obj, (list, tuple)) and not any(
            tf.nest.is_nested(o) for o in obj
        )

    def _copy_object(self, obj):
        if isinstance(obj, metrics_mod.Metric):
            return obj.__class__.from_config(obj.get_config())
        return obj  # Can be a function or `None`.


def create_pseudo_output_names(outputs):
    """Create pseudo output names for a subclassed Model."""
    return _create_pseudo_names(outputs, prefix="output_")


def create_pseudo_input_names(inputs):
    """Create pseudo input names for a subclassed Model."""
    return _create_pseudo_names(inputs, prefix="input_")


def _create_pseudo_names(tensors, prefix):
    """Creates pseudo {input | output} names for subclassed Models.

    Warning: this function should only be used to define default
    names for `Metics` and `SavedModel`. No other use cases should
    rely on a `Model`'s input or output names.

    Example with dict:

    `{'a': [x1, x2], 'b': x3}` becomes:
    `['a_1', 'a_2', 'b']`

    Example with list:

    `[x, y]` becomes:
    `['output_1', 'output_2']`

    Args:
      tensors: `Model`'s outputs or inputs.
      prefix: 'output_' for outputs, 'input_' for inputs.

    Returns:
      Flattened list of pseudo names.
    """

    def one_index(ele):
        # Start with "output_1" instead of "output_0".
        if isinstance(ele, int):
            return ele + 1
        return ele

    flat_paths = list(tf.__internal__.nest.yield_flat_paths(tensors))
    flat_paths = tf.nest.map_structure(one_index, flat_paths)
    names = []
    for path in flat_paths:
        if not path:
            name = prefix + "1"  # Single output.
        else:
            name = "_".join(str(p) for p in path)
            if isinstance(path[0], int):
                name = prefix + name
        names.append(name)
    return names


def map_to_output_names(y_pred, output_names, struct):
    """Maps a dict to a list using `output_names` as keys.

    This is a convenience feature only. When a `Model`'s outputs
    are a list, you can specify per-output losses and metrics as
    a dict, where the keys are the output names. If you specify
    per-output losses and metrics via the same structure as the
    `Model`'s outputs (recommended), no mapping is performed.

    For the Functional API, the output names are the names of the
    last layer of each output. For the Subclass API, the output names
    are determined by `create_pseudo_output_names` (For example:
    `['output_1', 'output_2']` for a list of outputs).

    This mapping preserves backwards compatibility for `compile` and
    `fit`.

    Args:
      y_pred: Sample outputs of the Model, to determine if this convenience
        feature should be applied (`struct` is returned unmodified if `y_pred`
        isn't a flat list).
      output_names: List. The names of the outputs of the Model.
      struct: The structure to map.

    Returns:
      `struct` mapped to a list in same order as `output_names`.
    """
    single_output = not tf.nest.is_nested(y_pred)
    outputs_are_flat_list = (
        not single_output
        and isinstance(y_pred, (list, tuple))
        and not any(tf.nest.is_nested(y_p) for y_p in y_pred)
    )

    if (single_output or outputs_are_flat_list) and isinstance(struct, dict):
        output_names = output_names or create_pseudo_output_names(y_pred)
        struct = copy.copy(struct)
        new_struct = [struct.pop(name, None) for name in output_names]
        if struct:
            raise ValueError(
                "Found unexpected losses or metrics that do not correspond "
                f"to any Model output: {struct.keys()}. "
                f"Valid mode output names: {output_names}. "
                f"Received struct is: {struct}."
            )
        if len(new_struct) == 1:
            return new_struct[0]
        return new_struct
    else:
        return struct


def map_missing_dict_keys(y_pred, struct):
    """Replaces missing dict keys in `struct` with `None` placeholders."""
    if not isinstance(y_pred, dict) or not isinstance(struct, dict):
        return struct
    struct = copy.copy(struct)
    for k in y_pred.keys():
        if k not in struct:
            struct[k] = None
    return struct


def match_dtype_and_rank(y_t, y_p, sw):
    """Match dtype and rank of predictions."""
    if y_t.shape.rank == 1 and y_p.shape.rank == 2:
        y_t = tf.expand_dims(y_t, axis=-1)
    if sw is not None:
        if sw.shape.rank == 1 and y_p.shape.rank == 2:
            sw = tf.expand_dims(sw, axis=-1)

    # Dtype.
    # This is required mainly for custom loss functions which do not take care
    # casting dtypes.
    if (y_t.dtype.is_floating and y_p.dtype.is_floating) or (
        y_t.dtype.is_integer and y_p.dtype.is_integer
    ):
        y_t = tf.cast(y_t, y_p.dtype)

    if sw is not None:
        sw = tf.cast(sw, y_p.dtype)
    return y_t, y_p, sw


def get_custom_object_name(obj):
    """Returns the name to use for a custom loss or metric callable.

    Args:
      obj: Custom loss of metric callable

    Returns:
      Name to use, or `None` if the object was not recognized.
    """
    if hasattr(obj, "name"):  # Accept `Loss` instance as `Metric`.
        return obj.name
    elif hasattr(obj, "__name__"):  # Function.
        return obj.__name__
    elif hasattr(obj, "__class__"):  # Class instance.
        return generic_utils.to_snake_case(obj.__class__.__name__)
    else:  # Unrecognized object.
        return None