message: Update docs in optional_ops to refer to dataset elements
diff: @@ -34,14 +34,16 @@ from tensorflow.python.util.tf_export import tf_export
  @tf_export("data.experimental.Optional")
  @six.add_metaclass(abc.ABCMeta)
  class Optional(composite_tensor.CompositeTensor):
  - """Wraps a nested structure of tensors that may/may not be present at runtime.
  + """Wraps a value that may/may not be present at runtime.
  An `Optional` can represent the result of an operation that may fail as a
  value, rather than raising an exception and halting execution. For example,
  `tf.data.experimental.get_next_as_optional` returns an `Optional` that either
  contains the next value from a `tf.compat.v1.data.Iterator` if one exists, or
  - a "none"
  - value that indicates the end of the sequence has been reached.
  + a "none" value that indicates the end of the sequence has been reached.
  +
  + `Optional` can only be used by values that are convertible to `Tensor` or
  + `CompositeTensor`.
  """
  @abc.abstractmethod
  @@ -58,7 +60,7 @@ class Optional(composite_tensor.CompositeTensor):
  @abc.abstractmethod
  def get_value(self, name=None):
  - """Returns a nested structure of values wrapped by this optional.
  + """Returns the value wrapped by this optional.
  If this optional does not have a value (i.e. `self.has_value()` evaluates
  to `False`), this operation will raise `tf.errors.InvalidArgumentError`
  @@ -68,7 +70,7 @@ class Optional(composite_tensor.CompositeTensor):
  name: (Optional.) A name for the created operation.
  Returns:
  - A nested structure of `tf.Tensor` and/or `tf.SparseTensor` objects.
  + The wrapped value.
  """
  raise NotImplementedError("Optional.get_value()")
  @@ -87,7 +89,8 @@ class Optional(composite_tensor.CompositeTensor):
  """Returns an `Optional` that wraps the given value.
  Args:
  - value: A nested structure of `tf.Tensor` and/or `tf.SparseTensor` objects.
  + value: A value to wrap. The value must be convertible to `Tensor` or
  + `CompositeTensor`.
  Returns:
  An `Optional` that wraps `value`.
  