Tanh torch warnings
This pull request the warning generated by using torch.nn.functional.tanh (which is deprecated). This pull request changes it to torch.tanh to remove the warning.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3828?src=pr&el=h1) Report
> Merging [#3828](https://codecov.io/gh/huggingface/transformers/pull/3828?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/b1e2368b32f3af88a920dac47cfc02a869409b20&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3828/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/3828?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #3828   +/-   ##
=======================================
  Coverage   78.47%   78.47%           
=======================================
  Files         106      106           
  Lines       17924    17924           
=======================================
  Hits        14066    14066           
  Misses       3858     3858           
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3828?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/activations.py](https://codecov.io/gh/huggingface/transformers/pull/3828/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9hY3RpdmF0aW9ucy5weQ==) | `82.35% <ø> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/3828?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/3828?src=pr&el=footer). Last update [b1e2368...06daacd](https://codecov.io/gh/huggingface/transformers/pull/3828?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>LGTM