Enable Enum pickling/unpickling. (#43188)
Summary:
Stack from [ghstack](https://github.com/ezyang/ghstack):
* **https://github.com/pytorch/pytorch/issues/43188 Enable Enum pickling/unpickling.**
* https://github.com/pytorch/pytorch/issues/42963 Add Enum TorchScript serialization and deserialization support
* https://github.com/pytorch/pytorch/issues/42874 Fix enum constant printing and add FileCheck to all Enum tests
* https://github.com/pytorch/pytorch/issues/43121 Add Enum convert back to Python object support
Pull Request resolved: https://github.com/pytorch/pytorch/pull/43188
Reviewed By: zdevito
Differential Revision: D23365141
Pulled By: gmagogsfm
fbshipit-source-id: f0c93d4ac614dec047ad8640eb6bd9c74159b558
