[FSDP] Short-term fix to remove `optim_input` (#84201)
This is a short-term quick fix to accommodate using the existing optimizer state APIs without passing `optim_input`. It preserves the existing `optim_input` code path but if `optim_input` is `None` while `optim` is not, then the APIs will use the new code path that relies on `self.param_groups` to get the information previously provided by `optim_input`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/84201
Approved by: https://github.com/rohan-varma
