Enable multiprocessing in glue datasets
The preprocessing of glue datasets is too slow. This change enables multiprocessing to speed up the process of converting examples to features by utilizing multiple cpu cores.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4821?src=pr&el=h1) Report
> Merging [#4821](https://codecov.io/gh/huggingface/transformers/pull/4821?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/c58e6c129a153ca1a5021e5d7e642d00bf011e20&el=desc) will **increase** coverage by `1.54%`.
> The diff coverage is `88.88%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4821/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4821?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4821      +/-   ##
==========================================
+ Coverage   74.52%   76.07%   +1.54%     
==========================================
  Files         128      128              
  Lines       21497    21505       +8     
==========================================
+ Hits        16021    16360     +339     
+ Misses       5476     5145     -331     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4821?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/data/datasets/glue.py](https://codecov.io/gh/huggingface/transformers/pull/4821/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL2RhdGFzZXRzL2dsdWUucHk=) | `86.48% <88.88%> (+0.12%)` | :arrow_up: |
| [src/transformers/data/processors/glue.py](https://codecov.io/gh/huggingface/transformers/pull/4821/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL3Byb2Nlc3NvcnMvZ2x1ZS5weQ==) | `40.95% <0.00%> (-8.49%)` | :arrow_down: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4821/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `73.85% <0.00%> (-0.42%)` | :arrow_down: |
| [src/transformers/modeling\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/4821/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iZXJ0LnB5) | `86.42% <0.00%> (+0.18%)` | :arrow_up: |
| [src/transformers/modeling\_ctrl.py](https://codecov.io/gh/huggingface/transformers/pull/4821/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19jdHJsLnB5) | `98.64% <0.00%> (+2.71%)` | :arrow_up: |
| [src/transformers/modeling\_t5.py](https://codecov.io/gh/huggingface/transformers/pull/4821/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190NS5weQ==) | `83.26% <0.00%> (+6.29%)` | :arrow_up: |
| [src/transformers/configuration\_longformer.py](https://codecov.io/gh/huggingface/transformers/pull/4821/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2xvbmdmb3JtZXIucHk=) | `100.00% <0.00%> (+25.00%)` | :arrow_up: |
| [src/transformers/modeling\_longformer.py](https://codecov.io/gh/huggingface/transformers/pull/4821/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19sb25nZm9ybWVyLnB5) | `93.02% <0.00%> (+75.48%)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4821?src=pr&el=footer). Last update [c58e6c1...ef63cb8](https://codecov.io/gh/huggingface/transformers/pull/4821?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
