Use union to cast between incompatible function pointers (#38842)
Summary:
This fixes `can not cast between incompatible function types` error if code is compiled by gcc-9.3
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38842
Differential Revision: D21676360
Pulled By: malfet
fbshipit-source-id: d8b05d8381bfc961e06981731ebca87a516c2811
