[s2s] add BartTranslationDistiller for distilling mBART
Author: sshleifer
Pull request: yes

New class `BartTranslationDistiller` does the same distillation method as `SummarizationDistiller`, but computes BLEU scores instead of ROUGE scores. It also accepts `--src_lang` and `--tgt_lang` arguments from the command line.

There is one strong checkpoint already posted at `sshleifer/distillmbart-12-6/`. I will post more in the coming days.

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6363?src=pr&el=h1) Report
> Merging [#6363](https://codecov.io/gh/huggingface/transformers/pull/6363?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/be1520d3a3c09d729649c49fa3163bd938b6a238&el=desc) will **decrease** coverage by `1.55%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6363/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6363?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6363      +/-   ##
==========================================
- Coverage   79.93%   78.37%   -1.56%     
==========================================
  Files         153      148       -5     
  Lines       27888    27196     -692     
==========================================
- Hits        22293    21316     -977     
- Misses       5595     5880     +285     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6363?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_mobilebert.py](https://codecov.io/gh/huggingface/transformers/pull/6363/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9tb2JpbGViZXJ0LnB5) | `24.55% <0.00%> (-70.09%)` | :arrow_down: |
| [src/transformers/tokenization\_reformer.py](https://codecov.io/gh/huggingface/transformers/pull/6363/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fcmVmb3JtZXIucHk=) | `41.66% <0.00%> (-40.00%)` | :arrow_down: |
| [src/transformers/tokenization\_marian.py](https://codecov.io/gh/huggingface/transformers/pull/6363/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fbWFyaWFuLnB5) | `92.17% <0.00%> (-7.00%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_albert.py](https://codecov.io/gh/huggingface/transformers/pull/6363/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9hbGJlcnQucHk=) | `81.74% <0.00%> (-6.61%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/6363/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9lbGVjdHJhLnB5) | `94.24% <0.00%> (-1.84%)` | :arrow_down: |
| [src/transformers/modeling\_longformer.py](https://codecov.io/gh/huggingface/transformers/pull/6363/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19sb25nZm9ybWVyLnB5) | `89.21% <0.00%> (-1.64%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/6363/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9iZXJ0LnB5) | `95.48% <0.00%> (-1.10%)` | :arrow_down: |
| [src/transformers/configuration\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/6363/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2JhcnQucHk=) | `93.75% <0.00%> (-0.49%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/6363/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9hdXRvLnB5) | `66.66% <0.00%> (-0.21%)` | :arrow_down: |
| [src/transformers/tokenization\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/6363/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYmFydC5weQ==) | `95.77% <0.00%> (-0.18%)` | :arrow_down: |
| ... and [21 more](https://codecov.io/gh/huggingface/transformers/pull/6363/diff?src=pr&el=tree-more) | |

------

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