Fix two bugs on MNLI dataset and SST-2 respectively.
The text index of test data of SST-2 are 1 rather than 0.

The label of MNLI task has a tricky swap on MNLI dataset, which should also be involved in get_labels() of dataset for correctness. 
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4546?src=pr&el=h1) Report
> Merging [#4546](https://codecov.io/gh/huggingface/transformers/pull/4546?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/a34a9896ac2a4a33ff9cd805c76eed914c8d8965&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `42.85%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4546/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4546?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4546      +/-   ##
==========================================
- Coverage   77.87%   77.85%   -0.02%     
==========================================
  Files         123      123              
  Lines       20566    20568       +2     
==========================================
- Hits        16016    16014       -2     
- Misses       4550     4554       +4     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4546?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/data/processors/glue.py](https://codecov.io/gh/huggingface/transformers/pull/4546/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL3Byb2Nlc3NvcnMvZ2x1ZS5weQ==) | `49.44% <0.00%> (-0.19%)` | :arrow_down: |
| [src/transformers/data/datasets/glue.py](https://codecov.io/gh/huggingface/transformers/pull/4546/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL2RhdGFzZXRzL2dsdWUucHk=) | `86.36% <60.00%> (+0.20%)` | :arrow_up: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4546/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `73.44% <0.00%> (-0.42%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4546/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `88.50% <0.00%> (-0.17%)` | :arrow_down: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4546/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `90.41% <0.00%> (-0.12%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/4546?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4546?src=pr&el=footer). Last update [a34a989...6e60ed9](https://codecov.io/gh/huggingface/transformers/pull/4546?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>LGTM