[bart-mnli] Fix class flipping bug
Previously, `eval_mnli/acc` was 0.35, now .9. Same issue as other models ported from fairseq.
New results of Victor's command.
```bash
wandb:   eval_mnli-mm/acc 0.9001220504475184
wandb:           _runtime 274.7172155380249
wandb:          eval_loss 0.3496225342093929
wandb:      eval_mnli/acc 0.9011716760061131
wandb:         _timestamp 1592585765.2334569
```
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5141?src=pr&el=h1) Report
> Merging [#5141](https://codecov.io/gh/huggingface/transformers/pull/5141?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/e33929ef1e98a71ba7fc411f39cbb5451396ef02&el=desc) will **increase** coverage by `0.00%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5141/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5141?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #5141   +/-   ##
=======================================
  Coverage   77.19%   77.20%           
=======================================
  Files         133      133           
  Lines       22232    22233    +1     
=======================================
+ Hits        17162    17164    +2     
+ Misses       5070     5069    -1     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5141?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/data/datasets/glue.py](https://codecov.io/gh/huggingface/transformers/pull/5141/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL2RhdGFzZXRzL2dsdWUucHk=) | `86.56% <100.00%> (+0.20%)` | :arrow_up: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5141/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `75.00% <0.00%> (-0.41%)` | :arrow_down: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5141/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `91.28% <0.00%> (+0.12%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5141/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `85.96% <0.00%> (+0.14%)` | :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/5141?src=pr&el=footer). Last update [e33929e...70c5736](https://codecov.io/gh/huggingface/transformers/pull/5141?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
