Fix type annotations for state_dict() override (#55704)
Summary:
Change annotation to OrderedDict, but stringify it to stay compatible with Python-3.6
Fixes https://github.com/pytorch/pytorch/issues/55302
Pull Request resolved: https://github.com/pytorch/pytorch/pull/55704
Reviewed By: walterddr
Differential Revision: D27686011
Pulled By: malfet
fbshipit-source-id: 3a8dedf33f38d86767ebd4e8a1a8abfe850b375a
