[mBART] skip broken forward pass test, stronger integration test
(1) I fixed the config of mbart to go from BLEU score 12 to BLEU score 26. This requires that the `decoder_start_token_id=lang_code['ro']` but that we filter that out when we decode.
(2) I added another, harder, integration test case. The expected result is only 1 word different than GT. This also adds implicit coverage for batched generation.
(3) tried for awhile to fix the broken slow GPU `test_enro_forward`. 
On commits where the test passed, it doesn't pass any more. And the model hadn't been updated on S3 since March 26. So for now we skip it.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5327?src=pr&el=h1) Report
> Merging [#5327](https://codecov.io/gh/huggingface/transformers/pull/5327?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/efae6645e223f29cf05eeafe95105a9f869b66dd&el=desc) will **increase** coverage by `0.21%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5327/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5327?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5327      +/-   ##
==========================================
+ Coverage   77.69%   77.90%   +0.21%     
==========================================
  Files         138      138              
  Lines       24291    24292       +1     
==========================================
+ Hits        18872    18924      +52     
+ Misses       5419     5368      -51     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5327?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/5327/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYmFydC5weQ==) | `93.87% <100.00%> (+6.37%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/5327/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9kaXN0aWxiZXJ0LnB5) | `66.25% <0.00%> (-32.52%)` | :arrow_down: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5327/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `79.26% <0.00%> (-0.34%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5327/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `87.62% <0.00%> (+0.14%)` | :arrow_up: |
| [src/transformers/configuration\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5327/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX3V0aWxzLnB5) | `96.37% <0.00%> (+0.72%)` | :arrow_up: |
| [src/transformers/tokenization\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/5327/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYXV0by5weQ==) | `97.77% <0.00%> (+2.22%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/5327/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9lbGVjdHJhLnB5) | `95.38% <0.00%> (+68.46%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/5327?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/5327?src=pr&el=footer). Last update [1af58c0...e077948](https://codecov.io/gh/huggingface/transformers/pull/5327?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Looks good to me