Added SiLU activation function (#41034)
Summary:
Implemented the SiLU activation function as discussed in https://github.com/pytorch/pytorch/issues/3169.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/41034
Reviewed By: glaringlee
Differential Revision: D22465203
Pulled By: heitorschueroff
fbshipit-source-id: b27d064529fc99600c586ad49b594b52b718b0d2
