Adds predict stage for glue tasks, and generate result files which can be submitted to gluebenchmark.com
By simply fine-tune robterta-large with 3k steps on several tasks, achieved:


Task | Metrics | Score
-- | -- | --
Microsoft Research Paraphrase Corpus | F1 / Accuracy | 91.5/88.6
Semantic Textual Similarity Benchmark | Pearson-Spearman Corr | 90.7/90.2
Quora Question Pairs | F1 / Accuracy | 69.5/87.3
Recognizing Textual Entailment | Accuracy | 82.0
Winograd NLI | Accuracy | 65.1



Looks good!

I just improved consistency with other scripts we have (in particular, `run_ner.py`) by:
- using an enum instead of two boolean flags
- I also always append the actual label name in the predictions file, which removes the need for a new arg

Let me know if that works for you and I'll merge to master soon<|||||># [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4463?src=pr&el=h1) Report
> Merging [#4463](https://codecov.io/gh/huggingface/transformers/pull/4463?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/8f1d0471489352ec01556ae61f8e8246002bbc58&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `46.03%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4463/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4463?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4463      +/-   ##
==========================================
- Coverage   77.93%   77.91%   -0.02%     
==========================================
  Files         123      123              
  Lines       20430    20474      +44     
==========================================
+ Hits        15922    15953      +31     
- Misses       4508     4521      +13     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4463?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/data/processors/glue.py](https://codecov.io/gh/huggingface/transformers/pull/4463/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL3Byb2Nlc3NvcnMvZ2x1ZS5weQ==) | `49.62% <31.81%> (-1.41%)` | :arrow_down: |
| [src/transformers/data/datasets/glue.py](https://codecov.io/gh/huggingface/transformers/pull/4463/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL2RhdGFzZXRzL2dsdWUucHk=) | `86.15% <77.77%> (-4.05%)` | :arrow_down: |
| [src/transformers/data/processors/utils.py](https://codecov.io/gh/huggingface/transformers/pull/4463/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL3Byb2Nlc3NvcnMvdXRpbHMucHk=) | `27.63% <100.00%> (+0.47%)` | :arrow_up: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4463/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `90.41% <0.00%> (-0.12%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4463/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `88.50% <0.00%> (+1.64%)` | :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/4463?src=pr&el=footer). Last update [8f1d047...d172a3b](https://codecov.io/gh/huggingface/transformers/pull/4463?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Thanks Julien for the code improvement! This look very good to me. <|||||>Thank you for contributing this!