AutoTokenizer supports mbart-large-en-ro
Author: sshleifer
Pull request: yes

- adds `MBartConfig` so that `MBartTokenizer` can have it's own key in `TOKENIZER_MAPPING`.
- I will remove all mentions of `sshleifer/mbart-large-en-ro` right before this is merged.
It's only difference with `facebook/mbart-large-en-ro` is that `config.model_type='mbart'`. Since I didn't want to break existing Mbart callers, I will do the s3 migration right before I merge.


Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5121?src=pr&el=h1) Report
> Merging [#5121](https://codecov.io/gh/huggingface/transformers/pull/5121?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/355954ffca798bb81d9db8886e30ce10f11e8a40&el=desc) will **increase** coverage by `0.01%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5121/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5121?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5121      +/-   ##
==========================================
+ Coverage   77.28%   77.29%   +0.01%     
==========================================
  Files         133      133              
  Lines       22134    22136       +2     
==========================================
+ Hits        17107    17111       +4     
+ Misses       5027     5025       -2     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5121?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/configuration\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/5121/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2F1dG8ucHk=) | `93.02% <100.00%> (ø)` | |
| [src/transformers/configuration\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/5121/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2JhcnQucHk=) | `93.61% <100.00%> (+0.28%)` | :arrow_up: |
| [src/transformers/tokenization\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/5121/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYXV0by5weQ==) | `97.67% <100.00%> (ø)` | |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5121/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `91.06% <0.00%> (+0.12%)` | :arrow_up: |
| [src/transformers/trainer.py](https://codecov.io/gh/huggingface/transformers/pull/5121/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyLnB5) | `39.62% <0.00%> (+0.23%)` | :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/5121?src=pr&el=footer). Last update [355954f...65b8c8a](https://codecov.io/gh/huggingface/transformers/pull/5121?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).