Allow GradScaler to be pickled (#38296)
Summary:
Should unblock https://github.com/PyTorchLightning/pytorch-lightning/issues/1782.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38296
Differential Revision: D21553296
Pulled By: albanD
fbshipit-source-id: 9041a72d7cf8833e4b01bc767fd2321f17c7c5f2
