added type hints for BART model (#16270)

* added type hints for BART model

* make fixup, adding imports to copied files

* Adding some missing types to cookiecutter

* Adding some missing types to cookiecutter

* Adding some missing types to cookiecutter

Co-authored-by: matt <rocketknight1@gmail.com>

templates/adding_a_new_model/cookiecutter-template-{{cookiecutter.modelname}}/modeling_{{cookiecutter.lowercase_modelname}}.py

@@ -1571,7 +1571,7 @@ class {{cookiecutter.camelcase_modelname}}ForQuestionAnswering({{cookiecutter.ca
 import math
 import copy
 import random
-from typing import Optional, Tuple
+from typing import Optional, Tuple, List, Union
 
 import torch
 from torch import nn