add a constexpr in c10::Half (#16091)
Summary:
Debug build generates references which are not resolved otherwise
as recognized by dlibenzi
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16091
Differential Revision: D13703584
Pulled By: soumith
fbshipit-source-id: 6ac5666d2c6b1520e083f6eac9c535a1609d9c6b
