[mobilebert] Avoid F.tanh deprecation warning
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5413?src=pr&el=h1) Report
> Merging [#5413](https://codecov.io/gh/huggingface/transformers/pull/5413?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/ac611145926ff63ee6d6cbd0b28c19bacb6f7ea1&el=desc) will **increase** coverage by `0.44%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5413/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5413?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5413      +/-   ##
==========================================
+ Coverage   77.42%   77.87%   +0.44%     
==========================================
  Files         140      140              
  Lines       24334    24334              
==========================================
+ Hits        18841    18949     +108     
+ Misses       5493     5385     -108     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5413?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_mobilebert.py](https://codecov.io/gh/huggingface/transformers/pull/5413/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19tb2JpbGViZXJ0LnB5) | `88.90% <100.00%> (ø)` | |
| [src/transformers/modeling\_tf\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/5413/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9kaXN0aWxiZXJ0LnB5) | `66.25% <0.00%> (-32.52%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5413/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `86.43% <0.00%> (+0.25%)` | :arrow_up: |
| [src/transformers/tokenization\_albert.py](https://codecov.io/gh/huggingface/transformers/pull/5413/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYWxiZXJ0LnB5) | `87.50% <0.00%> (+58.65%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/5413/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9lbGVjdHJhLnB5) | `95.38% <0.00%> (+68.46%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
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