Add TF auto model to the docs + fix sphinx warnings (again)
Author: sgugger
Pull request: yes

The goal of this PR was just to add the automodel to the docs (as pointed out in #5145) but then I had 100 sphinx warnings that made me grumpy so I had to fix them (some of them from the auto model docs, but most of them from #4978). 

As usual a few of them were harmless, others had a real impact on the docs, so I think we should make sure in the CI that there are no sphinx warnings to avoid making the docs bad by mistake.

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5187?src=pr&el=h1) Report
> Merging [#5187](https://codecov.io/gh/huggingface/transformers/pull/5187?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/ebc36108dc1c20985905c79f7d6a00f57f3cd3ae&el=desc) will **decrease** coverage by `0.89%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5187/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5187?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5187      +/-   ##
==========================================
- Coverage   77.99%   77.10%   -0.90%     
==========================================
  Files         138      138              
  Lines       23772    23772              
==========================================
- Hits        18541    18329     -212     
- Misses       5231     5443     +212     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5187?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_albert.py](https://codecov.io/gh/huggingface/transformers/pull/5187/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19hbGJlcnQucHk=) | `80.43% <ø> (ø)` | |
| [src/transformers/modeling\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/5187/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iYXJ0LnB5) | `96.23% <ø> (ø)` | |
| [src/transformers/modeling\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/5187/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iZXJ0LnB5) | `87.50% <ø> (ø)` | |
| [src/transformers/modeling\_ctrl.py](https://codecov.io/gh/huggingface/transformers/pull/5187/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19jdHJsLnB5) | `99.14% <ø> (ø)` | |
| [src/transformers/modeling\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/5187/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19kaXN0aWxiZXJ0LnB5) | `97.70% <ø> (ø)` | |
| [src/transformers/modeling\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/5187/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19lbGVjdHJhLnB5) | `80.19% <ø> (ø)` | |
| [src/transformers/modeling\_flaubert.py](https://codecov.io/gh/huggingface/transformers/pull/5187/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19mbGF1YmVydC5weQ==) | `84.12% <ø> (ø)` | |
| [src/transformers/modeling\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/5187/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19ncHQyLnB5) | `85.43% <ø> (ø)` | |
| [src/transformers/modeling\_longformer.py](https://codecov.io/gh/huggingface/transformers/pull/5187/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19sb25nZm9ybWVyLnB5) | `93.04% <ø> (ø)` | |
| [src/transformers/modeling\_mobilebert.py](https://codecov.io/gh/huggingface/transformers/pull/5187/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19tb2JpbGViZXJ0LnB5) | `88.78% <ø> (ø)` | |
| ... and [24 more](https://codecov.io/gh/huggingface/transformers/pull/5187/diff?src=pr&el=tree-more) | |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5187?src=pr&el=footer). Last update [ebc3610...cca9865](https://codecov.io/gh/huggingface/transformers/pull/5187?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).