Add MBART to models exportable with ONNX (#13049)

* Add MBART to models exportable with ONNX

* unittest mock

* Add tests

* Misc fixes

src/transformers/testing_utils.py

@@ -25,6 +25,7 @@ from distutils.util import strtobool
 from io import StringIO
 from pathlib import Path
 from typing import Iterator, Union
+from unittest import mock
 
 from transformers import logging as transformers_logging
 
@@ -1007,7 +1008,7 @@ def mockenv(**kwargs):
         use_tf = os.getenv("USE_TF", False)
 
     """
-    return unittest.mock.patch.dict(os.environ, kwargs)
+    return mock.patch.dict(os.environ, kwargs)
 
 
 # from https://stackoverflow.com/a/34333710/9201239