Add GELU to activations.py VERSION 2

[SriRangaTarun]

Summary

Add GELU activation functions. Here are the fast and slow implementations.

Related Issues

This issue

PR Overview

Adds two GELU implementations : one fast and one slow to activations.py.

• [n] This PR requires new unit tests [y/n] (make sure tests are included)
• [y] This PR requires to update the documentation [y/n] (make sure the docs are up-to-date)
• [y] This PR is backwards compatible [y/n]
• [n] This PR changes the current API [y/n] (all API changes need to be approved by fchollet)

[Dref360]

I don't think this should be merged into Keras.

Not widely used
Not published yet
Please submit your PR at keras-contrib.

[gabrieldemarmiesse]

Well I agree. This is something that users can easily do themselves (it's a one liner) and even if this is used a lot, the layer should pass the test of time to be integrated in core keras. If users don't use it anymore in six months, that's not worth it.

Let's wait for now.