[FSDP][optim_state_dict] Make FSDP optim_state_dict aware of DDP prefix (#96415)
Summary: When wrapping FSDP within DDP, optimizer state_dict may be broken due to the prefix of DDP. This PR fixes the issue.
Test Plan: CI
Differential Revision: D43893609
Pull Request resolved: https://github.com/pytorch/pytorch/pull/96415
Approved by: https://github.com/zhaojuanmao
