Patch `Half.h` for compiling CUDA with clang (#29027)
Summary:
Following discussion: https://github.com/pytorch/pytorch/issues/28417
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29027
Differential Revision: D19698745
Pulled By: ezyang
fbshipit-source-id: fab4be3bcbac8f3b334d7e0a56e6a790e2c6b6d8
