Add Type Hints to modeling_utils.py Closes #3911 (#3948)
* Add Type Hints to modeling_utils.py Closes #3911
Add Type Hints to methods in `modeling_utils.py`
Note: The coverage isn't 100%. Mostly skipped internal methods.
* Reformat according to `black` and `isort`
* Use typing.Iterable instead of Sequence
* Parameterize Iterable by its generic type
* Use typing.Optional when None is the default value
* Adhere to style guideline
* Update src/transformers/modeling_utils.py
* Update src/transformers/modeling_utils.py
Co-authored-by: Julien Chaumond <chaumond@gmail.com>
