Remove state_dict from AveragedModel and use buffers instead (#71763)
Summary:
Fixes [https://github.com/pytorch/pytorch/issues/66686](https://github.com/pytorch/pytorch/issues/66686)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/71763
Reviewed By: anjali411
Differential Revision: D33770907
Pulled By: prabhat00155
fbshipit-source-id: ee32f2cb8475c9add4e1a9a5d3d784ef95825efc
(cherry picked from commit a15898b072ae5234c76afa005ec492ed158c51aa)
