Add Sigmoid operator from Caffe2 (#46286)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/46286
commonize fp16 unary operators
Reviewed By: hyuen
Differential Revision: D24199660
fbshipit-source-id: 99bffa24dc3fa459561a7a2743b1a4dce4be5d58
