[Don't merge - Bert2Bert] Add training scripts and slight changes to Trainer
Just a draft to keep track of Bert2Bert summary training.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5718?src=pr&el=h1) Report
> Merging [#5718](https://codecov.io/gh/huggingface/transformers/pull/5718?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/fa5423b1695cd24856bcff47214172e0f540d924&el=desc) will **decrease** coverage by `0.46%`.
> The diff coverage is `10.76%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5718/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5718?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5718      +/-   ##
==========================================
- Coverage   77.79%   77.33%   -0.47%     
==========================================
  Files         145      146       +1     
  Lines       25355    25413      +58     
==========================================
- Hits        19726    19652      -74     
- Misses       5629     5761     +132     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5718?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/bert\_encoder\_decoder\_summary.py](https://codecov.io/gh/huggingface/transformers/pull/5718/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9iZXJ0X2VuY29kZXJfZGVjb2Rlcl9zdW1tYXJ5LnB5) | `0.00% <0.00%> (ø)` | |
| [src/transformers/trainer.py](https://codecov.io/gh/huggingface/transformers/pull/5718/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/5718/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmluZ19hcmdzLnB5) | `77.77% <100.00%> (+0.22%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/5718/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9lbGVjdHJhLnB5) | `26.02% <0.00%> (-69.52%)` | :arrow_down: |
| [src/transformers/tokenization\_xlm\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/5718/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25feGxtX3JvYmVydGEucHk=) | `84.52% <0.00%> (-10.72%)` | :arrow_down: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5718/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `79.59% <0.00%> (+0.33%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/5718/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9yb2JlcnRhLnB5) | `93.36% <0.00%> (+49.37%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5718?src=pr&el=footer). Last update [fa5423b...d9f6d07](https://codecov.io/gh/huggingface/transformers/pull/5718?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
