Use constants from math.h for gelu op (#20974)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20974
Use constants from math.h for gelu op
Reviewed By: hl475, houseroad
Differential Revision: D15511736
fbshipit-source-id: 7d069888fb5c7c310774d056f18711365b39b8e4
