add first implementation of swish (#41085)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/41085
add the first LUT implementation of swish
Test Plan:
compared against swish lowered as x*sigmoid(x), had to
increase the threshold of error but looks generally right
Reviewed By: venkatacrc
Differential Revision: D22418117
fbshipit-source-id: c75fa496aa7a5356ddc87f1d61650f432e389457
