get_activation('relu') provides a simple mapping from strings in configs to activation functions
Happy to do TF in a separate PR. I don't think worth breaking backwards compatibility over this.<|||||># [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2807?src=pr&el=h1) Report
> Merging [#2807](https://codecov.io/gh/huggingface/transformers/pull/2807?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/70bbe4b1de298651a9665dc86ba9689bca1e080f?src=pr&el=desc) will **increase** coverage by `29.04%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2807/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2807?src=pr&el=tree)

```diff
@@             Coverage Diff             @@
##           master    #2807       +/-   ##
===========================================
+ Coverage   44.91%   73.96%   +29.04%     
===========================================
  Files          94       94               
  Lines       15274    15274               
===========================================
+ Hits         6860    11297     +4437     
+ Misses       8414     3977     -4437
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2807?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/activations.py](https://codecov.io/gh/huggingface/transformers/pull/2807/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9hY3RpdmF0aW9ucy5weQ==) | `92.85% <ø> (ø)` | |
| [src/transformers/data/processors/squad.py](https://codecov.io/gh/huggingface/transformers/pull/2807/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL3Byb2Nlc3NvcnMvc3F1YWQucHk=) | `28.89% <ø> (+0.64%)` | :arrow_up: |
| [src/transformers/modeling\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/2807/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ194bG5ldC5weQ==) | `70.86% <100%> (+70.86%)` | :arrow_up: |
| [src/transformers/modeling\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/2807/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19kaXN0aWxiZXJ0LnB5) | `97.62% <100%> (+97.62%)` | :arrow_up: |
| [src/transformers/modeling\_xlm.py](https://codecov.io/gh/huggingface/transformers/pull/2807/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ194bG0ucHk=) | `86.37% <100%> (+86.37%)` | :arrow_up: |
| [src/transformers/modeling\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/2807/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iZXJ0LnB5) | `88.16% <100%> (+88.16%)` | :arrow_up: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/2807/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `61.32% <100%> (+61.32%)` | :arrow_up: |
| [src/transformers/modeling\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/2807/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19ncHQyLnB5) | `83.28% <100%> (+83.28%)` | :arrow_up: |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/2807/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `80.2% <100%> (+80.2%)` | :arrow_up: |
| ... and [28 more](https://codecov.io/gh/huggingface/transformers/pull/2807/diff?src=pr&el=tree-more) | |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2807?src=pr&el=footer). Last update [70bbe4b...6879e76](https://codecov.io/gh/huggingface/transformers/pull/2807?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
