elmo.py

import json
import logging
import warnings
from typing import Any, Dict, List, Union

import numpy
import torch
from overrides import overrides
from torch.nn.modules import Dropout

from allennlp.common import FromParams
from allennlp.common.checks import ConfigurationError
from allennlp.common.file_utils import cached_path
from allennlp.common.util import lazy_groups_of
from allennlp.data.instance import Instance
from allennlp.data.tokenizers.token_class import Token
from allennlp.data.vocabulary import Vocabulary
from allennlp.data.batch import Batch
from allennlp.data.fields import TextField
from allennlp.data.token_indexers.elmo_indexer import (
    ELMoCharacterMapper,
    ELMoTokenCharactersIndexer,
)
from allennlp.modules.elmo_lstm import ElmoLstm
from allennlp.modules.highway import Highway
from allennlp.modules.scalar_mix import ScalarMix
from allennlp.nn.util import (
    add_sentence_boundary_token_ids,
    get_device_of,
    remove_sentence_boundaries,
)

with warnings.catch_warnings():
    warnings.filterwarnings("ignore", category=FutureWarning)
    import h5py


logger = logging.getLogger(__name__)


class Elmo(torch.nn.Module, FromParams):
    """
    Compute ELMo representations using a pre-trained bidirectional language model.

    See "Deep contextualized word representations", Peters et al. for details.

    This module takes character id input and computes `num_output_representations` different layers
    of ELMo representations.  Typically `num_output_representations` is 1 or 2.  For example, in
    the case of the SRL model in the above paper, `num_output_representations=1` where ELMo was included at
    the input token representation layer.  In the case of the SQuAD model, `num_output_representations=2`
    as ELMo was also included at the GRU output layer.

    In the implementation below, we learn separate scalar weights for each output layer,
    but only run the biLM once on each input sequence for efficiency.

    # Parameters

    options_file : `str`, required.
        ELMo JSON options file
    weight_file : `str`, required.
        ELMo hdf5 weight file
    num_output_representations : `int`, required.
        The number of ELMo representation to output with
        different linear weighted combination of the 3 layers (i.e.,
        character-convnet output, 1st lstm output, 2nd lstm output).
    requires_grad : `bool`, optional
        If True, compute gradient of ELMo parameters for fine tuning.
    do_layer_norm : `bool`, optional, (default = `False`).
        Should we apply layer normalization (passed to `ScalarMix`)?
    dropout : `float`, optional, (default = `0.5`).
        The dropout to be applied to the ELMo representations.
    vocab_to_cache : `List[str]`, optional, (default = `None`).
        A list of words to pre-compute and cache character convolutions
        for. If you use this option, Elmo expects that you pass word
        indices of shape (batch_size, timesteps) to forward, instead
        of character indices. If you use this option and pass a word which
        wasn't pre-cached, this will break.
    keep_sentence_boundaries : `bool`, optional, (default = `False`)
        If True, the representation of the sentence boundary tokens are
        not removed.
    scalar_mix_parameters : `List[float]`, optional, (default = `None`)
        If not `None`, use these scalar mix parameters to weight the representations
        produced by different layers. These mixing weights are not updated during
        training. The mixing weights here should be the unnormalized (i.e., pre-softmax)
        weights. So, if you wanted to use only the 1st layer of a 2-layer ELMo,
        you can set this to [-9e10, 1, -9e10 ].
    module : `torch.nn.Module`, optional, (default = `None`).
        If provided, then use this module instead of the pre-trained ELMo biLM.
        If using this option, then pass `None` for both `options_file`
        and `weight_file`.  The module must provide a public attribute
        `num_layers` with the number of internal layers and its `forward`
        method must return a `dict` with `activations` and `mask` keys
        (see `_ElmoBilm` for an example).  Note that `requires_grad` is also
        ignored with this option.
    """

    def __init__(
        self,
        options_file: str,
        weight_file: str,
        num_output_representations: int,
        requires_grad: bool = False,
        do_layer_norm: bool = False,
        dropout: float = 0.5,
        vocab_to_cache: List[str] = None,
        keep_sentence_boundaries: bool = False,
        scalar_mix_parameters: List[float] = None,
        module: torch.nn.Module = None,
    ) -> None:
        super().__init__()

        logger.info("Initializing ELMo")
        if module is not None:
            if options_file is not None or weight_file is not None:
                raise ConfigurationError("Don't provide options_file or weight_file with module")
            self._elmo_lstm = module
        else:
            self._elmo_lstm = _ElmoBiLm(  # type: ignore
                options_file,
                weight_file,
                requires_grad=requires_grad,
                vocab_to_cache=vocab_to_cache,
            )
        self._has_cached_vocab = vocab_to_cache is not None
        self._keep_sentence_boundaries = keep_sentence_boundaries
        self._dropout = Dropout(p=dropout)
        self._scalar_mixes: Any = []
        for k in range(num_output_representations):
            scalar_mix = ScalarMix(
                self._elmo_lstm.num_layers,  # type: ignore
                do_layer_norm=do_layer_norm,
                initial_scalar_parameters=scalar_mix_parameters,
                trainable=scalar_mix_parameters is None,
            )
            self.add_module("scalar_mix_{}".format(k), scalar_mix)
            self._scalar_mixes.append(scalar_mix)

    def get_output_dim(self):
        return self._elmo_lstm.get_output_dim()

    def forward(
        self, inputs: torch.Tensor, word_inputs: torch.Tensor = None
    ) -> Dict[str, Union[torch.Tensor, List[torch.Tensor]]]:
        """
        # Parameters

        inputs : `torch.Tensor`, required.
        Shape `(batch_size, timesteps, 50)` of character ids representing the current batch.
        word_inputs : `torch.Tensor`, required.
            If you passed a cached vocab, you can in addition pass a tensor of shape
            `(batch_size, timesteps)`, which represent word ids which have been pre-cached.

        # Returns

        `Dict[str, Union[torch.Tensor, List[torch.Tensor]]]`
            A dict with the following keys:
            - `'elmo_representations'` (`List[torch.Tensor]`) :
              A `num_output_representations` list of ELMo representations for the input sequence.
              Each representation is shape `(batch_size, timesteps, embedding_dim)`
            - `'mask'` (`torch.BoolTensor`) :
              Shape `(batch_size, timesteps)` long tensor with sequence mask.
        """
        # reshape the input if needed
        original_shape = inputs.size()
        if len(original_shape) > 3:
            timesteps, num_characters = original_shape[-2:]
            reshaped_inputs = inputs.view(-1, timesteps, num_characters)
        else:
            reshaped_inputs = inputs

        if word_inputs is not None:
            original_word_size = word_inputs.size()
            if self._has_cached_vocab and len(original_word_size) > 2:
                reshaped_word_inputs = word_inputs.view(-1, original_word_size[-1])
            elif not self._has_cached_vocab:
                logger.warning(
                    "Word inputs were passed to ELMo but it does not have a cached vocab."
                )
                reshaped_word_inputs = None
            else:
                reshaped_word_inputs = word_inputs
        else:
            reshaped_word_inputs = word_inputs

        # run the biLM
        bilm_output = self._elmo_lstm(reshaped_inputs, reshaped_word_inputs)  # type: ignore
        layer_activations = bilm_output["activations"]
        mask_with_bos_eos = bilm_output["mask"]

        # compute the elmo representations
        representations = []
        for i in range(len(self._scalar_mixes)):
            scalar_mix = getattr(self, "scalar_mix_{}".format(i))
            representation_with_bos_eos = scalar_mix(layer_activations, mask_with_bos_eos)
            if self._keep_sentence_boundaries:
                processed_representation = representation_with_bos_eos
                processed_mask = mask_with_bos_eos
            else:
                representation_without_bos_eos, mask_without_bos_eos = remove_sentence_boundaries(
                    representation_with_bos_eos, mask_with_bos_eos
                )
                processed_representation = representation_without_bos_eos
                processed_mask = mask_without_bos_eos
            representations.append(self._dropout(processed_representation))

        # reshape if necessary
        if word_inputs is not None and len(original_word_size) > 2:
            mask = processed_mask.view(original_word_size)
            elmo_representations = [
                representation.view(original_word_size + (-1,))
                for representation in representations
            ]
        elif len(original_shape) > 3:
            mask = processed_mask.view(original_shape[:-1])
            elmo_representations = [
                representation.view(original_shape[:-1] + (-1,))
                for representation in representations
            ]
        else:
            mask = processed_mask
            elmo_representations = representations

        return {"elmo_representations": elmo_representations, "mask": mask}


def batch_to_ids(batch: List[List[str]]) -> torch.Tensor:
    """
    Converts a batch of tokenized sentences to a tensor representing the sentences with encoded characters
    (len(batch), max sentence length, max word length).

    # Parameters

    batch : `List[List[str]]`, required
        A list of tokenized sentences.

    # Returns

        A tensor of padded character ids.
    """
    instances = []
    indexer = ELMoTokenCharactersIndexer()
    for sentence in batch:
        tokens = [Token(token) for token in sentence]
        field = TextField(tokens, {"character_ids": indexer})
        instance = Instance({"elmo": field})
        instances.append(instance)

    dataset = Batch(instances)
    vocab = Vocabulary()
    dataset.index_instances(vocab)
    return dataset.as_tensor_dict()["elmo"]["character_ids"]["elmo_tokens"]


class _ElmoCharacterEncoder(torch.nn.Module):
    """
    Compute context insensitive token representation using pretrained biLM.

    This embedder has input character ids of size (batch_size, sequence_length, 50)
    and returns (batch_size, sequence_length + 2, embedding_dim), where embedding_dim
    is specified in the options file (typically 512).

    We add special entries at the beginning and end of each sequence corresponding
    to <S> and </S>, the beginning and end of sentence tokens.

    Note: this is a lower level class useful for advanced usage.  Most users should
    use `ElmoTokenEmbedder` or `allennlp.modules.Elmo` instead.

    # Parameters

    options_file : `str`
        ELMo JSON options file
    weight_file : `str`
        ELMo hdf5 weight file
    requires_grad : `bool`, optional, (default = `False`).
        If True, compute gradient of ELMo parameters for fine tuning.


    The relevant section of the options file is something like:

    ```
    {'char_cnn': {
        'activation': 'relu',
        'embedding': {'dim': 4},
        'filters': [[1, 4], [2, 8], [3, 16], [4, 32], [5, 64]],
        'max_characters_per_token': 50,
        'n_characters': 262,
        'n_highway': 2
        }
    }
    ```
    """

    def __init__(self, options_file: str, weight_file: str, requires_grad: bool = False) -> None:
        super().__init__()

        with open(cached_path(options_file), "r") as fin:
            self._options = json.load(fin)
        self._weight_file = weight_file

        self.output_dim = self._options["lstm"]["projection_dim"]
        self.requires_grad = requires_grad

        self._load_weights()

        # Cache the arrays for use in forward -- +1 due to masking.
        self._beginning_of_sentence_characters = torch.from_numpy(
            numpy.array(ELMoCharacterMapper.beginning_of_sentence_characters) + 1
        )
        self._end_of_sentence_characters = torch.from_numpy(
            numpy.array(ELMoCharacterMapper.end_of_sentence_characters) + 1
        )

    def get_output_dim(self):
        return self.output_dim

    @overrides
    def forward(self, inputs: torch.Tensor) -> Dict[str, torch.Tensor]:
        """
        Compute context insensitive token embeddings for ELMo representations.

        # Parameters

        inputs : `torch.Tensor`
            Shape `(batch_size, sequence_length, 50)` of character ids representing the
            current batch.

        # Returns

        Dict with keys:
        `'token_embedding'` : `torch.Tensor`
            Shape `(batch_size, sequence_length + 2, embedding_dim)` tensor with context
            insensitive token representations.
        `'mask'`:  `torch.BoolTensor`
            Shape `(batch_size, sequence_length + 2)` long tensor with sequence mask.
        """
        # Add BOS/EOS
        mask = (inputs > 0).sum(dim=-1) > 0
        character_ids_with_bos_eos, mask_with_bos_eos = add_sentence_boundary_token_ids(
            inputs, mask, self._beginning_of_sentence_characters, self._end_of_sentence_characters
        )

        # the character id embedding
        max_chars_per_token = self._options["char_cnn"]["max_characters_per_token"]
        # (batch_size * sequence_length, max_chars_per_token, embed_dim)
        character_embedding = torch.nn.functional.embedding(
            character_ids_with_bos_eos.view(-1, max_chars_per_token), self._char_embedding_weights
        )

        # run convolutions
        cnn_options = self._options["char_cnn"]
        if cnn_options["activation"] == "tanh":
            activation = torch.tanh
        elif cnn_options["activation"] == "relu":
            activation = torch.nn.functional.relu
        else:
            raise ConfigurationError("Unknown activation")

        # (batch_size * sequence_length, embed_dim, max_chars_per_token)
        character_embedding = torch.transpose(character_embedding, 1, 2)
        convs = []
        for i in range(len(self._convolutions)):
            conv = getattr(self, "char_conv_{}".format(i))
            convolved = conv(character_embedding)
            # (batch_size * sequence_length, n_filters for this width)
            convolved, _ = torch.max(convolved, dim=-1)
            convolved = activation(convolved)
            convs.append(convolved)

        # (batch_size * sequence_length, n_filters)
        token_embedding = torch.cat(convs, dim=-1)

        # apply the highway layers (batch_size * sequence_length, n_filters)
        token_embedding = self._highways(token_embedding)

        # final projection  (batch_size * sequence_length, embedding_dim)
        token_embedding = self._projection(token_embedding)

        # reshape to (batch_size, sequence_length, embedding_dim)
        batch_size, sequence_length, _ = character_ids_with_bos_eos.size()

        return {
            "mask": mask_with_bos_eos,
            "token_embedding": token_embedding.view(batch_size, sequence_length, -1),
        }

    def _load_weights(self):
        self._load_char_embedding()
        self._load_cnn_weights()
        self._load_highway()
        self._load_projection()

    def _load_char_embedding(self):
        with h5py.File(cached_path(self._weight_file), "r") as fin:
            char_embed_weights = fin["char_embed"][...]

        weights = numpy.zeros(
            (char_embed_weights.shape[0] + 1, char_embed_weights.shape[1]), dtype="float32"
        )
        weights[1:, :] = char_embed_weights

        self._char_embedding_weights = torch.nn.Parameter(
            torch.FloatTensor(weights), requires_grad=self.requires_grad
        )

    def _load_cnn_weights(self):
        cnn_options = self._options["char_cnn"]
        filters = cnn_options["filters"]
        char_embed_dim = cnn_options["embedding"]["dim"]

        convolutions = []
        for i, (width, num) in enumerate(filters):
            conv = torch.nn.Conv1d(
                in_channels=char_embed_dim, out_channels=num, kernel_size=width, bias=True
            )
            # load the weights
            with h5py.File(cached_path(self._weight_file), "r") as fin:
                weight = fin["CNN"]["W_cnn_{}".format(i)][...]
                bias = fin["CNN"]["b_cnn_{}".format(i)][...]

            w_reshaped = numpy.transpose(weight.squeeze(axis=0), axes=(2, 1, 0))
            if w_reshaped.shape != tuple(conv.weight.data.shape):
                raise ValueError("Invalid weight file")
            conv.weight.data.copy_(torch.FloatTensor(w_reshaped))
            conv.bias.data.copy_(torch.FloatTensor(bias))

            conv.weight.requires_grad = self.requires_grad
            conv.bias.requires_grad = self.requires_grad

            convolutions.append(conv)
            self.add_module("char_conv_{}".format(i), conv)

        self._convolutions = convolutions

    def _load_highway(self):

        # the highway layers have same dimensionality as the number of cnn filters
        cnn_options = self._options["char_cnn"]
        filters = cnn_options["filters"]
        n_filters = sum(f[1] for f in filters)
        n_highway = cnn_options["n_highway"]

        # create the layers, and load the weights
        self._highways = Highway(n_filters, n_highway, activation=torch.nn.functional.relu)
        for k in range(n_highway):
            # The AllenNLP highway is one matrix multiplication with concatenation of
            # transform and carry weights.
            with h5py.File(cached_path(self._weight_file), "r") as fin:
                # The weights are transposed due to multiplication order assumptions in tf
                # vs pytorch (tf.matmul(X, W) vs pytorch.matmul(W, X))
                w_transform = numpy.transpose(fin["CNN_high_{}".format(k)]["W_transform"][...])
                # -1.0 since AllenNLP is g * x + (1 - g) * f(x) but tf is (1 - g) * x + g * f(x)
                w_carry = -1.0 * numpy.transpose(fin["CNN_high_{}".format(k)]["W_carry"][...])
                weight = numpy.concatenate([w_transform, w_carry], axis=0)
                self._highways._layers[k].weight.data.copy_(torch.FloatTensor(weight))
                self._highways._layers[k].weight.requires_grad = self.requires_grad

                b_transform = fin["CNN_high_{}".format(k)]["b_transform"][...]
                b_carry = -1.0 * fin["CNN_high_{}".format(k)]["b_carry"][...]
                bias = numpy.concatenate([b_transform, b_carry], axis=0)
                self._highways._layers[k].bias.data.copy_(torch.FloatTensor(bias))
                self._highways._layers[k].bias.requires_grad = self.requires_grad

    def _load_projection(self):
        cnn_options = self._options["char_cnn"]
        filters = cnn_options["filters"]
        n_filters = sum(f[1] for f in filters)

        self._projection = torch.nn.Linear(n_filters, self.output_dim, bias=True)
        with h5py.File(cached_path(self._weight_file), "r") as fin:
            weight = fin["CNN_proj"]["W_proj"][...]
            bias = fin["CNN_proj"]["b_proj"][...]
            self._projection.weight.data.copy_(torch.FloatTensor(numpy.transpose(weight)))
            self._projection.bias.data.copy_(torch.FloatTensor(bias))

            self._projection.weight.requires_grad = self.requires_grad
            self._projection.bias.requires_grad = self.requires_grad


class _ElmoBiLm(torch.nn.Module):
    """
    Run a pre-trained bidirectional language model, outputting the activations at each
    layer for weighting together into an ELMo representation (with
    `allennlp.modules.seq2seq_encoders.Elmo`).  This is a lower level class, useful
    for advanced uses, but most users should use `allennlp.modules.Elmo` directly.

    # Parameters

    options_file : `str`
        ELMo JSON options file
    weight_file : `str`
        ELMo hdf5 weight file
    requires_grad : `bool`, optional, (default = `False`).
        If True, compute gradient of ELMo parameters for fine tuning.
    vocab_to_cache : `List[str]`, optional, (default = `None`).
        A list of words to pre-compute and cache character convolutions
        for. If you use this option, _ElmoBiLm expects that you pass word
        indices of shape (batch_size, timesteps) to forward, instead
        of character indices. If you use this option and pass a word which
        wasn't pre-cached, this will break.
    """

    def __init__(
        self,
        options_file: str,
        weight_file: str,
        requires_grad: bool = False,
        vocab_to_cache: List[str] = None,
    ) -> None:
        super().__init__()

        self._token_embedder = _ElmoCharacterEncoder(
            options_file, weight_file, requires_grad=requires_grad
        )

        self._requires_grad = requires_grad
        if requires_grad and vocab_to_cache:
            logging.warning(
                "You are fine tuning ELMo and caching char CNN word vectors. "
                "This behaviour is not guaranteed to be well defined, particularly. "
                "if not all of your inputs will occur in the vocabulary cache."
            )
        # This is an embedding, used to look up cached
        # word vectors built from character level cnn embeddings.
        self._word_embedding = None
        self._bos_embedding: torch.Tensor = None
        self._eos_embedding: torch.Tensor = None
        if vocab_to_cache:
            logging.info("Caching character cnn layers for words in vocabulary.")
            # This sets 3 attributes, _word_embedding, _bos_embedding and _eos_embedding.
            # They are set in the method so they can be accessed from outside the
            # constructor.
            self.create_cached_cnn_embeddings(vocab_to_cache)

        with open(cached_path(options_file), "r") as fin:
            options = json.load(fin)
        if not options["lstm"].get("use_skip_connections"):
            raise ConfigurationError("We only support pretrained biLMs with residual connections")
        self._elmo_lstm = ElmoLstm(
            input_size=options["lstm"]["projection_dim"],
            hidden_size=options["lstm"]["projection_dim"],
            cell_size=options["lstm"]["dim"],
            num_layers=options["lstm"]["n_layers"],
            memory_cell_clip_value=options["lstm"]["cell_clip"],
            state_projection_clip_value=options["lstm"]["proj_clip"],
            requires_grad=requires_grad,
        )
        self._elmo_lstm.load_weights(weight_file)
        # Number of representation layers including context independent layer
        self.num_layers = options["lstm"]["n_layers"] + 1

    def get_output_dim(self):
        return 2 * self._token_embedder.get_output_dim()

    def forward(
        self, inputs: torch.Tensor, word_inputs: torch.Tensor = None
    ) -> Dict[str, Union[torch.Tensor, List[torch.Tensor]]]:
        """
        # Parameters

        inputs : `torch.Tensor`, required.
            Shape `(batch_size, timesteps, 50)` of character ids representing the current batch.
        word_inputs : `torch.Tensor`, required.
            If you passed a cached vocab, you can in addition pass a tensor of shape `(batch_size, timesteps)`,
            which represent word ids which have been pre-cached.

        # Returns

        Dict with keys:

        `'activations'` : `List[torch.Tensor]`
            A list of activations at each layer of the network, each of shape
            `(batch_size, timesteps + 2, embedding_dim)`
        `'mask'`:  `torch.BoolTensor`
            Shape `(batch_size, timesteps + 2)` long tensor with sequence mask.

        Note that the output tensors all include additional special begin and end of sequence
        markers.
        """
        if self._word_embedding is not None and word_inputs is not None:
            try:
                mask_without_bos_eos = word_inputs > 0
                # The character cnn part is cached - just look it up.
                embedded_inputs = self._word_embedding(word_inputs)  # type: ignore
                # shape (batch_size, timesteps + 2, embedding_dim)
                type_representation, mask = add_sentence_boundary_token_ids(
                    embedded_inputs, mask_without_bos_eos, self._bos_embedding, self._eos_embedding
                )
            except (RuntimeError, IndexError):
                # Back off to running the character convolutions,
                # as we might not have the words in the cache.
                token_embedding = self._token_embedder(inputs)
                mask = token_embedding["mask"]
                type_representation = token_embedding["token_embedding"]
        else:
            token_embedding = self._token_embedder(inputs)
            mask = token_embedding["mask"]
            type_representation = token_embedding["token_embedding"]
        lstm_outputs = self._elmo_lstm(type_representation, mask)

        # Prepare the output.  The first layer is duplicated.
        # Because of minor differences in how masking is applied depending
        # on whether the char cnn layers are cached, we'll be defensive and
        # multiply by the mask here. It's not strictly necessary, as the
        # mask passed on is correct, but the values in the padded areas
        # of the char cnn representations can change.
        output_tensors = [
            torch.cat([type_representation, type_representation], dim=-1) * mask.unsqueeze(-1)
        ]
        for layer_activations in torch.chunk(lstm_outputs, lstm_outputs.size(0), dim=0):
            output_tensors.append(layer_activations.squeeze(0))

        return {"activations": output_tensors, "mask": mask}

    def create_cached_cnn_embeddings(self, tokens: List[str]) -> None:
        """
        Given a list of tokens, this method precomputes word representations
        by running just the character convolutions and highway layers of elmo,
        essentially creating uncontextual word vectors. On subsequent forward passes,
        the word ids are looked up from an embedding, rather than being computed on
        the fly via the CNN encoder.

        This function sets 3 attributes:

        _word_embedding : `torch.Tensor`
            The word embedding for each word in the tokens passed to this method.
        _bos_embedding : `torch.Tensor`
            The embedding for the BOS token.
        _eos_embedding : `torch.Tensor`
            The embedding for the EOS token.

        # Parameters

        tokens : `List[str]`, required.
            A list of tokens to precompute character convolutions for.
        """
        tokens = [ELMoCharacterMapper.bos_token, ELMoCharacterMapper.eos_token] + tokens
        timesteps = 32
        batch_size = 32
        chunked_tokens = lazy_groups_of(iter(tokens), timesteps)

        all_embeddings = []
        device = get_device_of(next(self.parameters()))
        for batch in lazy_groups_of(chunked_tokens, batch_size):
            # Shape (batch_size, timesteps, 50)
            batched_tensor = batch_to_ids(batch)
            # NOTE: This device check is for when a user calls this method having
            # already placed the model on a device. If this is called in the
            # constructor, it will probably happen on the CPU. This isn't too bad,
            # because it's only a few convolutions and will likely be very fast.
            if device >= 0:
                batched_tensor = batched_tensor.cuda(device)
            output = self._token_embedder(batched_tensor)
            token_embedding = output["token_embedding"]
            mask = output["mask"]
            token_embedding, _ = remove_sentence_boundaries(token_embedding, mask)
            all_embeddings.append(token_embedding.view(-1, token_embedding.size(-1)))
        full_embedding = torch.cat(all_embeddings, 0)

        # We might have some trailing embeddings from padding in the batch, so
        # we clip the embedding and lookup to the right size.
        full_embedding = full_embedding[: len(tokens), :]
        embedding = full_embedding[2 : len(tokens), :]
        vocab_size, embedding_dim = list(embedding.size())

        from allennlp.modules.token_embedders import Embedding  # type: ignore

        self._bos_embedding = full_embedding[0, :]
        self._eos_embedding = full_embedding[1, :]
        self._word_embedding = Embedding(  # type: ignore
            num_embeddings=vocab_size,
            embedding_dim=embedding_dim,
            weight=embedding.data,
            trainable=self._requires_grad,
            padding_index=0,
        )