Only use F.gelu for torch >=1.4.0
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2955?src=pr&el=h1) Report
> Merging [#2955](https://codecov.io/gh/huggingface/transformers/pull/2955?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/3e98f27e4a9bb0ac3d0fe24b94d30da42cdae8a7?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `66.66%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2955/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2955?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##           master   #2955      +/-   ##
=========================================
- Coverage    76.1%   76.1%   -0.01%     
=========================================
  Files          98      98              
  Lines       15946   15948       +2     
=========================================
+ Hits        12136   12137       +1     
- Misses       3810    3811       +1
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2955?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/activations.py](https://codecov.io/gh/huggingface/transformers/pull/2955/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9hY3RpdmF0aW9ucy5weQ==) | `87.5% <66.66%> (-5.36%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/2955?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2955?src=pr&el=footer). Last update [3e98f27...49c66bd](https://codecov.io/gh/huggingface/transformers/pull/2955?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
