message: Add NotImplementedError message for fit, evaluate and predict generator
  functions.
diff: @@ -1884,6 +1884,10 @@ class Model(Network):
  Raises:
  ValueError: In case the generator yields data in an invalid format.
  """
  + if self._distribution_strategy:
  + raise NotImplementedError('`fit_generator` is not supported for '
  + 'models compiled with DistributionStrategy.')
  +
  if not self.built and not self._is_graph_network:
  raise NotImplementedError(
  '`fit_generator` is not yet enabled for unbuilt Model subclasses')
  @@ -1951,6 +1955,10 @@ class Model(Network):
  Raises:
  ValueError: In case the generator yields data in an invalid format.
  """
  + if self._distribution_strategy:
  + raise NotImplementedError('`evaluate_generator` is not supported for '
  + 'models compiled with DistributionStrategy.')
  +
  if not self.built and not self._is_graph_network:
  raise NotImplementedError(
  '`evaluate_generator` is not yet enabled for '
  @@ -2005,6 +2013,10 @@ class Model(Network):
  Raises:
  ValueError: In case the generator yields data in an invalid format.
  """
  + if self._distribution_strategy:
  + raise NotImplementedError('`predict_generator` is not supported for '
  + 'models compiled with DistributionStrategy.')
  +
  if not self.built and not self._is_graph_network:
  raise NotImplementedError(
  '`predict_generator` is not yet enabled for unbuilt Model subclasses')
  