Fix glue processor failing on tf datasets
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2564?src=pr&el=h1) Report
> Merging [#2564](https://codecov.io/gh/huggingface/transformers/pull/2564?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/6d5049a24d5906ece3fd9b68fb3abe1a0b6bb049?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `0%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2564/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2564?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #2564      +/-   ##
==========================================
- Coverage    74.6%   74.58%   -0.02%     
==========================================
  Files          87       87              
  Lines       14802    14805       +3     
==========================================
  Hits        11043    11043              
- Misses       3759     3762       +3
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2564?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/data/processors/glue.py](https://codecov.io/gh/huggingface/transformers/pull/2564/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL3Byb2Nlc3NvcnMvZ2x1ZS5weQ==) | `27.53% <0%> (-0.34%)` | :arrow_down: |

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<|||||>A proper fix for this would probably be to add a unit test that sends tf datasets through GLUE. Let me know if you want me to add that in as well.<|||||>That's great, thanks @neonbjb 