Fix all sphynx warnings
This PR touches a lot of files but nothing that should break anything, it's just there to fix all sphynx warnings. Why? Well some of them are just there to annoy us and don't have any effect, but for roughly half of them, there is something wrong going on in the docs so it's best to fix them all, especially to make it easier to spot new warnings introduced when writing new docs.

The only thing that is a real change is that I removed `members` in the `AdamWeightDecay` because it was documenting its `apply_gradients` method using the docstring from keras which is not sphynx-compatible (it was not rendering properly in our docs). If we really want that method documented (it was the only one), I can rewrite the docstring.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5068?src=pr&el=h1) Report
> Merging [#5068](https://codecov.io/gh/huggingface/transformers/pull/5068?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/439aa1d6e9c953069f75fc23c737221d0df2c977&el=desc) will **increase** coverage by `0.98%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5068/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5068?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5068      +/-   ##
==========================================
+ Coverage   76.45%   77.43%   +0.98%     
==========================================
  Files         130      130              
  Lines       22024    22024              
==========================================
+ Hits        16839    17055     +216     
+ Misses       5185     4969     -216     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5068?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/configuration\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/5068/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2F1dG8ucHk=) | `92.85% <ø> (ø)` | |
| [src/transformers/configuration\_t5.py](https://codecov.io/gh/huggingface/transformers/pull/5068/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX3Q1LnB5) | `96.42% <ø> (ø)` | |
| [src/transformers/configuration\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/5068/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX3hsbmV0LnB5) | `94.00% <ø> (ø)` | |
| [src/transformers/data/processors/utils.py](https://codecov.io/gh/huggingface/transformers/pull/5068/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL3Byb2Nlc3NvcnMvdXRpbHMucHk=) | `27.63% <ø> (ø)` | |
| [src/transformers/modeling\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/5068/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19hdXRvLnB5) | `70.58% <ø> (ø)` | |
| [src/transformers/modeling\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/5068/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19lbGVjdHJhLnB5) | `78.16% <ø> (ø)` | |
| [src/transformers/modeling\_longformer.py](https://codecov.io/gh/huggingface/transformers/pull/5068/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19sb25nZm9ybWVyLnB5) | `93.02% <ø> (ø)` | |
| [src/transformers/modeling\_reformer.py](https://codecov.io/gh/huggingface/transformers/pull/5068/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19yZWZvcm1lci5weQ==) | `88.19% <ø> (ø)` | |
| [src/transformers/modeling\_tf\_albert.py](https://codecov.io/gh/huggingface/transformers/pull/5068/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9hbGJlcnQucHk=) | `75.33% <ø> (ø)` | |
| [src/transformers/modeling\_tf\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/5068/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9lbGVjdHJhLnB5) | `91.28% <ø> (ø)` | |
| ... and [16 more](https://codecov.io/gh/huggingface/transformers/pull/5068/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/5068?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5068?src=pr&el=footer). Last update [439aa1d...6a52a4b](https://codecov.io/gh/huggingface/transformers/pull/5068?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
