Turn the Half::from_bits into a constexpr function to avoid unresolve… (#17661)
Summary:
…d symbol errors when building in DEBUG mode.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17661
Differential Revision: D14319610
Pulled By: soumith
fbshipit-source-id: 6c508a37155e29260f403d7174f343aa1ff32385
