Make kernel registration constexpr again (#16166)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16166
Since we now don't use std::function anymore, we can make kernel registration constexpr again.
Reviewed By: ezyang
Differential Revision: D13738630
fbshipit-source-id: 918fa3a3c8c6f0ddbd0f08b3b143cdf066265387
