message: disallow calling fit
diff: @@ -43,6 +43,12 @@ class FunctionalPreprocessingStage(base_preprocessing_layer.PreprocessingLayer,
  trainable: Boolean, whether the model's variables should be trainable.
  """
  + def fit(self, *args, **kwargs):
  + raise ValueError('Preprocessing stage is not a complete model, and hence '
  + 'should not be `fit`. Instead, you may feed data to'
  + '`adapt` the stage to set appropriate states of the '
  + 'layers in the stage.')
  +
  def adapt(self, data, reset_state=True):
  """Adapt the state of the layers of the preprocessing stage to the data.
  @@ -113,11 +119,14 @@ class FunctionalPreprocessingStage(base_preprocessing_layer.PreprocessingLayer,
  def _unzip_dataset(ds):
  """Unzip dataset into a list of single element datasets.
  +
  Arguments:
  data: A Dataset object.
  +
  Return:
  A list of Dataset object, each correspond to one of the
  `element_spec` of the input Dataset object.
  +
  Example:
  >>> ds1 = tf.data.Dataset.from_tensor_slices([1, 2, 3])
  >>> ds2 = tf.data.Dataset.from_tensor_slices([4, 5, 6])
  