Model utils doc
This PR documents all the model utilities and fixes a few things in the `PreTrainedModel`/`TFPreTrainedModel`.

It introduces a new category I called "internal" that comes at the very end of the package documentation, which is aimed at documenting all our public internal methods (e.g., public in their modules, but not in the project init). This should be useful for people who tweak our code and reuse those pieces.

Preview of the new file in the docs is [here](https://64241-155220641-gh.circle-artifacts.com/0/docs/_build/html/internal/modeling_utils.html) (and you can navigate on the other pages if you want to check the smaller changes).
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6005?src=pr&el=h1) Report
> Merging [#6005](https://codecov.io/gh/huggingface/transformers/pull/6005?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/7e251ae0395bd3e558c633fe8664fbdf612cb4f4&el=desc) will **decrease** coverage by `0.00%`.
> The diff coverage is `95.83%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6005/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6005?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6005      +/-   ##
==========================================
- Coverage   78.66%   78.66%   -0.01%     
==========================================
  Files         146      146              
  Lines       26230    26231       +1     
==========================================
  Hits        20634    20634              
- Misses       5596     5597       +1     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6005?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/configuration\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6005/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX3V0aWxzLnB5) | `96.52% <ø> (ø)` | |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6005/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `88.96% <90.90%> (ø)` | |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6005/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `87.20% <100.00%> (+0.02%)` | :arrow_up: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6005/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `86.21% <0.00%> (-0.26%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/6005?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6005?src=pr&el=footer). Last update [7e251ae...ead6b62](https://codecov.io/gh/huggingface/transformers/pull/6005?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
