[FSDP][optim_state_dict] Fix a memory leakage in optim_state_dict (#96263)
Summary: The original code uses a class variable to store flat_parameter result. This could cause memory leakage.
Test Plan: CI and a E2E run
Reviewed By: awgu
Differential Revision: D43893577
Pull Request resolved: https://github.com/pytorch/pytorch/pull/96263
Approved by: https://github.com/zhaojuanmao
