clean code to unify the logic to use fp16 by the optimizer engine (#20915)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20915
Clean the unary processor code. Some question are added into the comments to seek suggestions.
Reviewed By: pjh5
Differential Revision: D15448502
fbshipit-source-id: ef0c45718c1a06187e3fe2e4e59b7f20c641d9c5
