Fix less_than_lowest warnings (#67422)
Summary:
Fixes useless comparison against zero warnings for Half.h
Pull Request resolved: https://github.com/pytorch/pytorch/pull/67422
Test Plan: Sandcastle
Reviewed By: ngimel
Differential Revision: D31951939
fbshipit-source-id: 3e9940adda2d57b4d9b122f3862706c673f9ef4b
