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 (and some I wasn't sure of).
I think this is good for merge, no? @julien-c <|||||># [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3948?src=pr&el=h1) Report
> Merging [#3948](https://codecov.io/gh/huggingface/transformers/pull/3948?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/7b75aa9fa55bee577e2c7403301ed31103125a35&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3948/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/3948?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3948      +/-   ##
==========================================
- Coverage   78.39%   78.38%   -0.02%     
==========================================
  Files         120      120              
  Lines       19925    19925              
==========================================
- Hits        15620    15618       -2     
- Misses       4305     4307       +2     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3948?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3948/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `90.96% <100.00%> (-0.13%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3948/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `92.77% <0.00%> (-0.17%)` | :arrow_down: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3948?src=pr&el=footer). Last update [7b75aa9...99848e8](https://codecov.io/gh/huggingface/transformers/pull/3948?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Thanks @bglearning!