Change to call if type_spec not provided

Signed-off-by: Cheng Su <scnju13@gmail.com>

python/ray/data/iterator.py

@@ -772,32 +772,45 @@ def to_tf(
             A ``tf.data.Dataset`` that yields inputs and targets.
         """  # noqa: E501
 
-        from ray.air._internal.tensorflow_utils import convert_ndarray_to_tf_tensor
+        from ray.air._internal.tensorflow_utils import (
+            convert_ndarray_to_tf_tensor,
+            get_type_spec,
+        )
 
         try:
             import tensorflow as tf
         except ImportError:
             raise ValueError("tensorflow must be installed!")
 
+        def validate_column(column: str) -> None:
+            if column not in valid_columns:
+                raise ValueError(
+                    f"You specified '{column}' in `feature_columns` or "
+                    f"`label_columns`, but there's no column named '{column}' in the "
+                    f"dataset. Valid column names are: {valid_columns}."
+                )
+
+        def validate_columns(columns: Union[str, List]) -> None:
+            if isinstance(columns, list):
+                for column in columns:
+                    validate_column(column)
+            else:
+                validate_column(columns)
+
         def convert_batch_to_tensors(
             batch: Dict[str, np.ndarray],
             *,
             columns: Union[str, List[str]],
-            type_spec: Union[tf.TypeSpec, Dict[str, tf.TypeSpec]] = None,
+            type_spec: Union[tf.TypeSpec, Dict[str, tf.TypeSpec]],
         ) -> Union[tf.Tensor, Dict[str, tf.Tensor]]:
             if isinstance(columns, str):
                 return convert_ndarray_to_tf_tensor(batch[columns], type_spec=type_spec)
-            else:
-                tensors = {}
-                for column in columns:
-                    if type_spec is not None:
-                        column_type_spec = type_spec[column]
-                    else:
-                        column_type_spec = None
-                    tensors[column] = convert_ndarray_to_tf_tensor(
-                        batch[column], type_spec=column_type_spec
-                    )
-                return tensors
+            return {
+                column: convert_ndarray_to_tf_tensor(
+                    batch[column], type_spec=type_spec[column]
+                )
+                for column in columns
+            }
 
         def generator():
             for batch in self.iter_batches(
@@ -817,13 +830,16 @@ def generator():
                 )
                 yield features, labels
 
-        if feature_type_spec is not None and label_type_spec is not None:
-            output_signature = (feature_type_spec, label_type_spec)
-        else:
-            output_signature = None
+        if feature_type_spec is None or label_type_spec is None:
+            schema = self.schema()
+            valid_columns = schema.names
+            validate_columns(feature_columns)
+            validate_columns(label_columns)
+            feature_type_spec = get_type_spec(schema, columns=feature_columns)
+            label_type_spec = get_type_spec(schema, columns=label_columns)
 
         dataset = tf.data.Dataset.from_generator(
-            generator, output_signature=output_signature
+            generator, output_signature=(feature_type_spec, label_type_spec)
         )
 
         options = tf.data.Options()