Multilingual BART -
This adds `mbart-en-ro` model, a BART variant finetuned on english-romanian translation.
### TODO
- [x] (docs) pretrained_model.rst
- [ ] (docs) README.md
- [ ] (docs) bart.rst


### Differences with Bart
`config.normalize_before`: all the `LayerNorm` calls happen before attention calls
`config.add_final_layer_norm`: There is one extra layer_norm in the decoder
`config.scale_embedding`: embeddings are multiplied by 32 (`sqrt(d_model=1024)`)

### Future PRs
- The model returns the same variables as fairseq, but the tokenizer is not yet at parity with fairseq. This is the next PR in the pipeline.
- the `mbart-large-cc25` (no finetuning) model has a very different state dict. Also WIP.

### Misc
- the link_tester got angry about me not typing out URLs in this PR. Unclear why it didn't happen earlier.
Needs documentation but unclear where to put it.

# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3602?src=pr&el=h1) Report
> Merging [#3602](https://codecov.io/gh/huggingface/transformers/pull/3602?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/1a16d9d94a81e95463b166adfce4a8e02cdc47eb&el=desc) will **not change** coverage by `%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3602/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/3602?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #3602   +/-   ##
=======================================
  Coverage   78.06%   78.06%           
=======================================
  Files         100      100           
  Lines       17181    17181           
=======================================
  Hits        13413    13413           
  Misses       3768     3768           
```



------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/3602?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/3602?src=pr&el=footer). Last update [1a16d9d...1a16d9d](https://codecov.io/gh/huggingface/transformers/pull/3602?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Hi, is there any reason that cc-25 was removed and only the fine-tuned one is kept? any way I can quickly enable that? thanks<|||||>When the authors released the CC25 checkpoint, it was shaped differently than `mbart-large-en-ro` and I am not clear on whether that is fixed yet.
See https://github.com/pytorch/fairseq/issues?q=is%3Aissue+is%3Aopen+sort%3Aupdated-desc+mbart