Generation doc
Author: sgugger
Pull request: yes

Add documentation (and clean docstrings) of `GenerationMixin` and `TFGenerationMixin`.

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6470?src=pr&el=h1) Report
> Merging [#6470](https://codecov.io/gh/huggingface/transformers/pull/6470?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/05810cd80a5ca83065e0dbe5335c030c4a435ddb&el=desc) will **decrease** coverage by `0.17%`.
> The diff coverage is `96.73%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6470/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6470?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6470      +/-   ##
==========================================
- Coverage   80.55%   80.37%   -0.18%     
==========================================
  Files         153      156       +3     
  Lines       28001    28058      +57     
==========================================
- Hits        22556    22552       -4     
- Misses       5445     5506      +61     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6470?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/configuration\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/6470/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2JhcnQucHk=) | `93.87% <ø> (-0.36%)` | :arrow_down: |
| [src/transformers/configuration\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6470/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX3V0aWxzLnB5) | `96.59% <ø> (ø)` | |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6470/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `86.46% <ø> (ø)` | |
| [src/transformers/tokenization\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/6470/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYmFydC5weQ==) | `100.00% <ø> (+4.05%)` | :arrow_up: |
| [src/transformers/modeling\_encoder\_decoder.py](https://codecov.io/gh/huggingface/transformers/pull/6470/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19lbmNvZGVyX2RlY29kZXIucHk=) | `91.66% <87.50%> (+0.64%)` | :arrow_up: |
| [src/transformers/tokenization\_mbart.py](https://codecov.io/gh/huggingface/transformers/pull/6470/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fbWJhcnQucHk=) | `95.31% <95.31%> (ø)` | |
| [src/transformers/modeling\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/6470/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19ncHQyLnB5) | `86.68% <97.87%> (+0.71%)` | :arrow_up: |
| [src/transformers/\_\_init\_\_.py](https://codecov.io/gh/huggingface/transformers/pull/6470/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9fX2luaXRfXy5weQ==) | `99.28% <100.00%> (+0.01%)` | :arrow_up: |
| [src/transformers/configuration\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/6470/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2F1dG8ucHk=) | `93.47% <100.00%> (+0.14%)` | :arrow_up: |
| [src/transformers/configuration\_mbart.py](https://codecov.io/gh/huggingface/transformers/pull/6470/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX21iYXJ0LnB5) | `100.00% <100.00%> (ø)` | |
| ... and [11 more](https://codecov.io/gh/huggingface/transformers/pull/6470/diff?src=pr&el=tree-more) | |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6470?src=pr&el=footer). Last update [05810cd...d9cbc03](https://codecov.io/gh/huggingface/transformers/pull/6470?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).