[HANS] Fix label_list for RoBERTa/BART (class flipping)
Author: VictorSanh
Pull request: yes

the MNLI checkpoints ported from fairseq (more specifically RoBERTa and BART) has flipping classes. @sshleifer  fixed it for `run_glue.py` last week (see https://github.com/huggingface/transformers/pull/5141), this PR fixes the same problem for `run_hans.py`.

Now the HANS evaluation for [bart-large-mnli](https://huggingface.co/facebook/bart-large-mnli) gives:
```
Heuristic entailed results:
lexical_overlap: 0.9932
subsequence: 0.9996
constituent: 0.9982

Heuristic non-entailed results:
lexical_overlap: 0.8002
subsequence: 0.263
constituent: 0.2074
```
 
For [roberta-large-mnli](https://huggingface.co/roberta-large-mnli):
```
Heuristic entailed results:
lexical_overlap: 1.0
subsequence: 1.0
constituent: 1.0

Heuristic non-entailed results:
lexical_overlap: 0.8746
subsequence: 0.2804
constituent: 0.1056
```

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5196?src=pr&el=h1) Report
> Merging [#5196](https://codecov.io/gh/huggingface/transformers/pull/5196?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/c439752482759c94784e11a87dcbf08ce69dccf3&el=desc) will **decrease** coverage by `0.08%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5196/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5196?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5196      +/-   ##
==========================================
- Coverage   78.07%   77.99%   -0.09%     
==========================================
  Files         138      138              
  Lines       23786    23786              
==========================================
- Hits        18572    18552      -20     
- Misses       5214     5234      +20     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5196?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/data/processors/squad.py](https://codecov.io/gh/huggingface/transformers/pull/5196/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL3Byb2Nlc3NvcnMvc3F1YWQucHk=) | `28.66% <0.00%> (-5.42%)` | :arrow_down: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5196/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `76.42% <0.00%> (-0.39%)` | :arrow_down: |
| [src/transformers/modeling\_transfo\_xl.py](https://codecov.io/gh/huggingface/transformers/pull/5196/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190cmFuc2ZvX3hsLnB5) | `78.61% <0.00%> (-0.20%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5196/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `85.86% <0.00%> (-0.15%)` | :arrow_down: |

------

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LGTM, though if we could do it again we would just flip the final layer weights when converting models... (discussed recently with @LysandreJik and @thomwolf)