Add serialization support for slots and subclass getstate/setstate (#62745)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/62745
Test Plan: Imported from OSS
Reviewed By: ezyang
Differential Revision: D30113112
Pulled By: albanD
fbshipit-source-id: 6c562d0c060fb0280e5e3d432bb42fb833e6d500
