Tf trainer cleanup
Author: sgugger
Pull request: yes

This follows up from #5982 and does the same thing for `TFTrainer`.
Basically no code is changed, just moved around and the customization points are made public and explicit to the user. Starting to make the docs better but I will do it properly (for both Trainer and TFTrainer) after this PR is merged.

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6015?src=pr&el=h1) Report
> Merging [#6015](https://codecov.io/gh/huggingface/transformers/pull/6015?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/3b44aa935a4d8f1b0e93a23070d97be6b9c9506b&el=desc) will **increase** coverage by `1.29%`.
> The diff coverage is `22.22%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6015/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6015?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6015      +/-   ##
==========================================
+ Coverage   77.32%   78.61%   +1.29%     
==========================================
  Files         146      146              
  Lines       26253    26270      +17     
==========================================
+ Hits        20299    20652     +353     
+ Misses       5954     5618     -336     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6015?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/trainer.py](https://codecov.io/gh/huggingface/transformers/pull/6015/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyLnB5) | `40.87% <ø> (ø)` | |
| [src/transformers/trainer\_tf.py](https://codecov.io/gh/huggingface/transformers/pull/6015/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyX3RmLnB5) | `16.24% <22.22%> (-0.30%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/6015/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9yb2JlcnRhLnB5) | `43.98% <0.00%> (-49.38%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/6015/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9kaXN0aWxiZXJ0LnB5) | `64.90% <0.00%> (-33.90%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6015/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `83.95% <0.00%> (-2.26%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/6015/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9lbGVjdHJhLnB5) | `95.53% <0.00%> (+69.51%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_mobilebert.py](https://codecov.io/gh/huggingface/transformers/pull/6015/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9tb2JpbGViZXJ0LnB5) | `96.77% <0.00%> (+73.38%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/6015?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/6015?src=pr&el=footer). Last update [3b44aa9...5ff70d6](https://codecov.io/gh/huggingface/transformers/pull/6015?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


As long as you don't create too many merge conflicts, I can wait ;-)

PR available #6038 

Merge conflicts were a mess so opened a new version.