Move prediction_loss_only to TrainingArguments
It didn't make sense to me to have that boolean flag in the init of `Trainer` when all the other ones are in `TrainingArguments` so I deprecated it and moved it.

Let me know if you think it's a wrong move.

Unrelated changes: had to fix `make quality` that was complaining about non-documented or tested models and it was easier to fix them than change my setup (which doesn't let me push if make quality fails).
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6426?src=pr&el=h1) Report
> Merging [#6426](https://codecov.io/gh/huggingface/transformers/pull/6426?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/66fa8ceaeaa6fe12f1bd4a5e6b0a924f59f715d9&el=desc) will **decrease** coverage by `2.62%`.
> The diff coverage is `36.36%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6426/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6426?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6426      +/-   ##
==========================================
- Coverage   79.90%   77.28%   -2.63%     
==========================================
  Files         153      153              
  Lines       27877    27884       +7     
==========================================
- Hits        22276    21549     -727     
- Misses       5601     6335     +734     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6426?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/trainer\_tf.py](https://codecov.io/gh/huggingface/transformers/pull/6426/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyX3RmLnB5) | `12.25% <0.00%> (-0.13%)` | :arrow_down: |
| [src/transformers/trainer.py](https://codecov.io/gh/huggingface/transformers/pull/6426/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyLnB5) | `37.84% <60.00%> (-0.03%)` | :arrow_down: |
| [src/transformers/training\_args.py](https://codecov.io/gh/huggingface/transformers/pull/6426/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmluZ19hcmdzLnB5) | `80.58% <100.00%> (+0.19%)` | :arrow_up: |
| [src/transformers/optimization.py](https://codecov.io/gh/huggingface/transformers/pull/6426/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9vcHRpbWl6YXRpb24ucHk=) | `28.94% <0.00%> (-67.11%)` | :arrow_down: |
| [src/transformers/pipelines.py](https://codecov.io/gh/huggingface/transformers/pull/6426/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9waXBlbGluZXMucHk=) | `26.98% <0.00%> (-52.81%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/6426/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9kaXN0aWxiZXJ0LnB5) | `34.11% <0.00%> (-30.36%)` | :arrow_down: |
| [src/transformers/optimization\_tf.py](https://codecov.io/gh/huggingface/transformers/pull/6426/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9vcHRpbWl6YXRpb25fdGYucHk=) | `33.33% <0.00%> (-24.33%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/6426/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9hdXRvLnB5) | `48.79% <0.00%> (-18.08%)` | :arrow_down: |
| [src/transformers/modeling\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/6426/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19hdXRvLnB5) | `64.16% <0.00%> (-14.46%)` | :arrow_down: |
| [src/transformers/data/processors/squad.py](https://codecov.io/gh/huggingface/transformers/pull/6426/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL3Byb2Nlc3NvcnMvc3F1YWQucHk=) | `13.76% <0.00%> (-14.38%)` | :arrow_down: |
| ... and [13 more](https://codecov.io/gh/huggingface/transformers/pull/6426/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/6426?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/6426?src=pr&el=footer). Last update [66fa8ce...34cca14](https://codecov.io/gh/huggingface/transformers/pull/6426?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>no strong opinion on this (but as usual consider the BC/cleanliness ratio carefully)