[FSDP][5/N] Lift `FSDPParamInfo` to use `FlatParamHandle` (#97665)
This PR changes `FSDPParamInfo` in `_optim_utils.py` to save the `FlatParamHandle`, not directly the `FlatParameter`. This is in preparation for subsequent PRs.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/97665
Approved by: https://github.com/rohan-varma
