if_constexpr for C++14 (#31091)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/31091
This implements a C++17 "if constexpr" like feature for C++14.
This can be used, for example, to replace SFINAE or to force the compiler to remove some parts of a function in the assembly based on a condition.
PRs stacked on top will use this to simplify some of our template metaprogramming.
ghstack-source-id: 102867141
Test Plan: unit tests
Differential Revision: D18927220
fbshipit-source-id: 19a135e00af6ebb0139ce3730353762d4512158f
