message: refine by using iterator of partitioned variable
diff: @@ -358,7 +358,7 @@ def _add_elements_to_collection(elements, collection_list):
  def _should_add_regularizer(variable, existing_variable_set):
  result = True
  if isinstance(variable, tf_variables.PartitionedVariable):
  - for var in variable._get_variable_list():
  + for var in variable:
  if var in existing_variable_set:
  result = False
  break
  