Add a script to check all models are tested and documented
Author: sgugger
Pull request: yes

This PR adds a new script to `make quality` that checks all models are tested and documented. This way, we will get a CI failure is someone adds a new model but doesn't document it or add it to the tests so that the common tests are applied to it. This will make the library far more robust.

Note: the changes in the doc files are related to models that were not documented. The changes in model files/test files are related to models that were not tested, had a bug and the corresponding fixes. I then got lazy and stopped trying to fix all the models not tested and just added TODO.

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6298?src=pr&el=h1) Report
> Merging [#6298](https://codecov.io/gh/huggingface/transformers/pull/6298?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/31da35cc8939e19a292cb7f15cad5c9a1ddf7f23&el=desc) will **decrease** coverage by `0.00%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6298/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6298?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6298      +/-   ##
==========================================
- Coverage   79.64%   79.64%   -0.01%     
==========================================
  Files         147      147              
  Lines       27120    27121       +1     
==========================================
  Hits        21600    21600              
- Misses       5520     5521       +1     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6298?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/6298/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19lbGVjdHJhLnB5) | `82.13% <ø> (+0.57%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_albert.py](https://codecov.io/gh/huggingface/transformers/pull/6298/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9hbGJlcnQucHk=) | `81.74% <100.00%> (+6.45%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/6298/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9kaXN0aWxiZXJ0LnB5) | `64.47% <0.00%> (-32.95%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6298/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `86.71% <0.00%> (+0.25%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/6298/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9ncHQyLnB5) | `94.70% <0.00%> (+22.94%)` | :arrow_up: |
| [src/transformers/tokenization\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/6298/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYmFydC5weQ==) | `95.77% <0.00%> (+35.21%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/6298?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/6298?src=pr&el=footer). Last update [31da35c...2cfe35e](https://codecov.io/gh/huggingface/transformers/pull/6298?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).