TORCH_FN (#39823)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39823
Add a compile time function pointer that can be used to pass function pointers in template args.
This is very useful for metaprogramming function wrappers.
ghstack-source-id: 105944072
Test Plan: waitforsandcastle
Differential Revision: D21986243
fbshipit-source-id: a123571c18aa0e65908cbb131f28922ceb59061c
