[FSDP] Add state_dict() save/reload in parity test (#73366)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/73366
Adds state_dict() save/reload in parity with DDP test to ensure
checkpointing doesn't cause issue with accuracy/model params.
ghstack-source-id: 150114251
Test Plan: CI
Reviewed By: fegin
Differential Revision: D34434358
fbshipit-source-id: fb0787486b383cfcbec7cc1325a486c8d9b1e2ea
(cherry picked from commit e3bcc7733cb5a497a640007044b1138dfee3a532)
