[FSDP][2/N] Rename "flattened parameter" -> "flat parameter" (pt. 2) (#97662)
From our recent experience, we refer to FSDP's `FlatParameter` as "flat parameter", not "flattened parameter". This PR renames that in `_optim_utils.py`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/97662
Approved by: https://github.com/rohan-varma
