Unboxing uses if_constexpr instead of SFINAE (#38145)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38145
Now that if_constexpr is landed, we can make this more readable
ghstack-source-id: 103765920
Test Plan: waitforsandcastle
Differential Revision: D21480798
fbshipit-source-id: 8181d4731036373cc3a1868fd6f4baeebb426081
