[WIP - Don't merge][EncoderDecoder] Extend Trainer for Bert2Bert
This PR draft should be used to train a Bert2Bert model. 
It's not ready to be merged yet.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5840?src=pr&el=h1) Report
> Merging [#5840](https://codecov.io/gh/huggingface/transformers/pull/5840?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/fa5423b1695cd24856bcff47214172e0f540d924&el=desc) will **decrease** coverage by `1.26%`.
> The diff coverage is `41.17%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5840/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5840?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5840      +/-   ##
==========================================
- Coverage   77.79%   76.53%   -1.27%     
==========================================
  Files         145      145              
  Lines       25355    25365      +10     
==========================================
- Hits        19726    19414     -312     
- Misses       5629     5951     +322     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5840?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/trainer.py](https://codecov.io/gh/huggingface/transformers/pull/5840/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyLnB5) | `37.41% <37.50%> (-0.55%)` | :arrow_down: |
| [src/transformers/training\_args.py](https://codecov.io/gh/huggingface/transformers/pull/5840/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmluZ19hcmdzLnB5) | `77.77% <100.00%> (+0.22%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_mobilebert.py](https://codecov.io/gh/huggingface/transformers/pull/5840/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9tb2JpbGViZXJ0LnB5) | `23.38% <0.00%> (-73.39%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/5840/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9yb2JlcnRhLnB5) | `93.36% <0.00%> (+49.37%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/5840?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/5840?src=pr&el=footer). Last update [fa5423b...d9f6d07](https://codecov.io/gh/huggingface/transformers/pull/5840?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
