Use hash to clean the test dirs
Author: JetRunner
Pull request: yes

This one solves it once for all. What do you think? @sgugger @LysandreJik 

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6475?src=pr&el=h1) Report
> Merging [#6475](https://codecov.io/gh/huggingface/transformers/pull/6475?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/05810cd80a5ca83065e0dbe5335c030c4a435ddb&el=desc) will **decrease** coverage by `1.12%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6475/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6475?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6475      +/-   ##
==========================================
- Coverage   80.55%   79.42%   -1.13%     
==========================================
  Files         153      153              
  Lines       28001    28001              
==========================================
- Hits        22556    22241     -315     
- Misses       5445     5760     +315     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6475?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_mobilebert.py](https://codecov.io/gh/huggingface/transformers/pull/6475/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9tb2JpbGViZXJ0LnB5) | `24.55% <0.00%> (-72.36%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6475/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `84.21% <0.00%> (-2.26%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6475/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `86.31% <0.00%> (-0.98%)` | :arrow_down: |
| [src/transformers/pipelines.py](https://codecov.io/gh/huggingface/transformers/pull/6475/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9waXBlbGluZXMucHk=) | `79.94% <0.00%> (+0.25%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/6475/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9ncHQyLnB5) | `95.01% <0.00%> (+23.16%)` | :arrow_up: |
| [src/transformers/tokenization\_albert.py](https://codecov.io/gh/huggingface/transformers/pull/6475/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYWxiZXJ0LnB5) | `87.50% <0.00%> (+58.65%)` | :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/6475?src=pr&el=footer). Last update [05810cd...646a7dc](https://codecov.io/gh/huggingface/transformers/pull/6475?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


Agreed, thanks for the fix!