message: Erroring when _is_pipelined is called before building a subclassed model
  Summary: TF2.5 Only
  Reviewers: #tensorflow, #framework_ip_review_-_any_oss_or_third-party_code_use_has_been_approved, georgep
  Maniphest Tasks: T59232
diff: @@ -123,6 +123,9 @@ class ModelExtension(keras_extension_base.KerasExtensionBase): # pylint: disabl
  # pylint: enable=protected-access
  def _is_pipelined(self):
  + assert self._graph_network_initialized, \
  + "`Model` subclasses cannot determine if they have pipeline stage " \
  + "assignments until they have been built."
  return bool(self._pipeline_stage_assignment)
  def _get_config_supported(self):
  