add pl_glue example test
Author: stas00
Pull request: yes

Currently PL glue example isn't being tested by CI. This PR fixes that.
https://github.com/huggingface/transformers/pull/6027

the added test currently fails (goodness) and fixes are being PR'ed in 
1. https://github.com/huggingface/transformers/pull/6027
2. https://github.com/huggingface/transformers/pull/6028

At the moment this is a basic 'can-run' test - I will add an actual validation once the `pl_glue_run.py` script is made to work. 


Comments:
Looks like this PR needs one of these merged first: https://github.com/huggingface/transformers/pull/6028 or https://github.com/huggingface/transformers/pull/6027, as `-n_gpus` is now required in `lightning_base.py`

No matter what I try CI can't get to acc/f1>=0.75, even though I get 1.0 on my machine. Suggestions?

OK, as suggested by @sshleifer I changed the PR to only test that `run_pl_glue.py` is able to run and complete its job. Will have to figure out quality testing another time. It's very inefficient to try to tune something that works just fine on my machine but fails on CI.

# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6034?src=pr&el=h1) Report
> Merging [#6034](https://codecov.io/gh/huggingface/transformers/pull/6034?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/1bbc54a87c293761823ff1c1833a4f77353af9a9&el=desc) will **decrease** coverage by `0.05%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6034/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6034?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6034      +/-   ##
==========================================
- Coverage   79.53%   79.47%   -0.06%     
==========================================
  Files         148      148              
  Lines       27196    27196              
==========================================
- Hits        21630    21614      -16     
- Misses       5566     5582      +16     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6034?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/6034/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fcm9iZXJ0YS5weQ==) | `76.71% <0.00%> (-21.92%)` | :arrow_down: |
| [src/transformers/tokenization\_utils\_base.py](https://codecov.io/gh/huggingface/transformers/pull/6034/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHNfYmFzZS5weQ==) | `86.43% <0.00%> (-7.42%)` | :arrow_down: |
| [src/transformers/tokenization\_utils\_fast.py](https://codecov.io/gh/huggingface/transformers/pull/6034/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHNfZmFzdC5weQ==) | `92.14% <0.00%> (-2.15%)` | :arrow_down: |
| [src/transformers/tokenization\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/6034/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fb3BlbmFpLnB5) | `82.57% <0.00%> (-1.52%)` | :arrow_down: |
| [src/transformers/tokenization\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/6034/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYmVydC5weQ==) | `91.07% <0.00%> (-0.45%)` | :arrow_down: |
| [src/transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6034/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHMucHk=) | `90.00% <0.00%> (-0.41%)` | :arrow_down: |
| [src/transformers/tokenization\_transfo\_xl.py](https://codecov.io/gh/huggingface/transformers/pull/6034/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdHJhbnNmb194bC5weQ==) | `38.73% <0.00%> (+5.16%)` | :arrow_up: |
| [src/transformers/tokenization\_xlm\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/6034/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25feGxtX3JvYmVydGEucHk=) | `95.23% <0.00%> (+10.71%)` | :arrow_up: |
| [src/transformers/tokenization\_marian.py](https://codecov.io/gh/huggingface/transformers/pull/6034/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fbWFyaWFuLnB5) | `92.17% <0.00%> (+25.21%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/6034?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/6034?src=pr&el=footer). Last update [1bbc54a...bf693dd](https://codecov.io/gh/huggingface/transformers/pull/6034?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


CI failure is totally unrelated.