Cleanup glue
Author: jysohn23
Pull request: yes

* Make sure that MNLI acc metrics are named differently for match vs mismatch
* Allow writing to `cache_dir` from Glue dataset in case where datasets live in readOnly filesystem
* Flush TB writer at end to not miss any metrics

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4621?src=pr&el=h1) Report
> Merging [#4621](https://codecov.io/gh/huggingface/transformers/pull/4621?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/003c4771290b00e6d14b871210c3a369edccaeed&el=desc) will **increase** coverage by `0.00%`.
> The diff coverage is `33.33%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4621/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4621?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #4621   +/-   ##
=======================================
  Coverage   78.03%   78.04%           
=======================================
  Files         124      124           
  Lines       20626    20627    +1     
=======================================
+ Hits        16096    16098    +2     
+ Misses       4530     4529    -1     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4621?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/data/datasets/glue.py](https://codecov.io/gh/huggingface/transformers/pull/4621/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL2RhdGFzZXRzL2dsdWUucHk=) | `86.15% <ø> (ø)` | |
| [src/transformers/data/metrics/\_\_init\_\_.py](https://codecov.io/gh/huggingface/transformers/pull/4621/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL21ldHJpY3MvX19pbml0X18ucHk=) | `26.66% <0.00%> (ø)` | |
| [src/transformers/trainer.py](https://codecov.io/gh/huggingface/transformers/pull/4621/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyLnB5) | `39.62% <100.00%> (+0.14%)` | :arrow_up: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4621/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `73.44% <0.00%> (-0.42%)` | :arrow_down: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4621/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `90.53% <0.00%> (+0.11%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4621/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `88.99% <0.00%> (+0.16%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/4621?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4621?src=pr&el=footer). Last update [003c477...01a48df](https://codecov.io/gh/huggingface/transformers/pull/4621?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


@julien-c bump..!