Clean up autoclass doc
Author: sgugger
Pull request: yes

Clean up and standardize the documentation of the auto classes.

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/7081?src=pr&el=h1) Report
> Merging [#7081](https://codecov.io/gh/huggingface/transformers/pull/7081?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/ae736163d0d7a3a167ff0df3bf6c824437bbba2a?el=desc) will **increase** coverage by `0.09%`.
> The diff coverage is `89.47%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/7081/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/7081?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #7081      +/-   ##
==========================================
+ Coverage   79.44%   79.54%   +0.09%     
==========================================
  Files         168      168              
  Lines       32260    32285      +25     
==========================================
+ Hits        25630    25680      +50     
+ Misses       6630     6605      -25     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/7081?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/configuration\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/7081/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2F1dG8ucHk=) | `96.10% <ø> (ø)` | |
| [src/transformers/modeling\_tf\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/7081/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9hdXRvLnB5) | `72.36% <ø> (+1.77%)` | :arrow_up: |
| [src/transformers/tokenization\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/7081/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYXV0by5weQ==) | `91.80% <ø> (ø)` | |
| [src/transformers/trainer.py](https://codecov.io/gh/huggingface/transformers/pull/7081/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyLnB5) | `54.90% <71.42%> (+0.22%)` | :arrow_up: |
| [src/transformers/modeling\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/7081/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19hdXRvLnB5) | `82.29% <100.00%> (+1.07%)` | :arrow_up: |
| [src/transformers/data/data\_collator.py](https://codecov.io/gh/huggingface/transformers/pull/7081/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL2RhdGFfY29sbGF0b3IucHk=) | `93.54% <0.00%> (+0.35%)` | :arrow_up: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/7081/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `86.46% <0.00%> (+5.76%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/7081?src=pr&el=footer). Last update [4cbd50e...dad4fcd](https://codecov.io/gh/huggingface/transformers/pull/7081?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).