Mark tests in TFAutoModelTest as slow.
Author: aaugustin
Pull request: yes

Each test forces downloading the same 536MB file, which is slow
even with a decent internet connection.

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2014?src=pr&el=h1) Report
> Merging [#2014](https://codecov.io/gh/huggingface/transformers/pull/2014?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/b0ee7c7df3d49a819c4d6cef977214bd91f5c075?src=pr&el=desc) will **decrease** coverage by `0.39%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2014/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2014?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##           master    #2014     +/-   ##
=========================================
- Coverage   84.05%   83.66%   -0.4%     
=========================================
  Files         105      105             
  Lines       15555    15555             
=========================================
- Hits        13075    13014     -61     
- Misses       2480     2541     +61
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2014?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [transformers/tests/modeling\_tf\_auto\_test.py](https://codecov.io/gh/huggingface/transformers/pull/2014/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL21vZGVsaW5nX3RmX2F1dG9fdGVzdC5weQ==) | `36.36% <100%> (-61.82%)` | :arrow_down: |
| [transformers/modeling\_tf\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/2014/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3RmX2F1dG8ucHk=) | `32.5% <0%> (-18.75%)` | :arrow_down: |
| [transformers/configuration\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/2014/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL2NvbmZpZ3VyYXRpb25fYXV0by5weQ==) | `45% <0%> (-15%)` | :arrow_down: |
| [transformers/configuration\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/2014/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL2NvbmZpZ3VyYXRpb25fdXRpbHMucHk=) | `88.31% <0%> (-3.9%)` | :arrow_down: |
| [transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/2014/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3RmX3V0aWxzLnB5) | `91.51% <0%> (-1.22%)` | :arrow_down: |
| [transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/2014/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL2ZpbGVfdXRpbHMucHk=) | `70% <0%> (-0.5%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/2014?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/2014?src=pr&el=footer). Last update [b0ee7c7...5ab9308](https://codecov.io/gh/huggingface/transformers/pull/2014?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


Predictably, this lowers code coverage, because CircleCI does coverage measurement without running the slow tests. Given that other tests with similar performance are skipped, I thought it would be consistent to skip these. If there's a specific reason for not doing so, I can document it in a comment instead.

I'm ok with that.