message: [tf.data] Expose `tf.contrib.data.Optional` and `tf.contrib.data.get_next_as_optional()`.
diff: @@ -26,6 +26,7 @@ See [Importing Data](https://tensorflow.org/guide/datasets) for an overview.
  @@CheckpointInputPipelineHook
  @@CsvDataset
  @@LMDBDataset
  +@@Optional
  @@RandomDataset
  @@Reducer
  @@SqlDataset
  @@ -38,7 +39,7 @@ See [Importing Data](https://tensorflow.org/guide/datasets) for an overview.
  @@copy_to_device
  @@dense_to_sparse_batch
  @@enumerate_dataset
  -
  +@@get_next_as_optional
  @@get_single_element
  @@group_by_reducer
  @@group_by_window
  @@ -46,7 +47,6 @@ See [Importing Data](https://tensorflow.org/guide/datasets) for an overview.
  @@make_batched_features_dataset
  @@make_csv_dataset
  @@make_saveable_from_iterator
  -
  @@map_and_batch
  @@padded_batch_and_drop_remainder
  @@parallel_interleave
  @@ -107,6 +107,8 @@ from tensorflow.contrib.data.python.ops.shuffle_ops import shuffle_and_repeat
  from tensorflow.contrib.data.python.ops.sliding import sliding_window_batch
  from tensorflow.contrib.data.python.ops.unique import unique
  from tensorflow.contrib.data.python.ops.writers import TFRecordWriter
  +from tensorflow.python.data.ops.iterator_ops import get_next_as_optional
  +from tensorflow.python.data.ops.optional_ops import Optional
  # pylint: enable=unused-import
  from tensorflow.python.util.all_util import remove_undocumented
  