implement GLU activation function

[ibeltagy]

Pick and implement one of the variants of the GLU activation.

Check here for comparison and equations
https://arxiv.org/pdf/2002.05202.pdf
https://arxiv.org/pdf/2102.11972.pdf

and here for implementation of one of them
https://github.com/EleutherAI/gpt-neox/blob/main/megatron/model/activations.py

[ibeltagy]

assigned to @jaketae

[jaketae]

Closed via #47, cc @ibeltagy