[FSDP][1/N] Rename "flattened parameter" to "flat parameter" (#97661)
From our recent experience, we refer to FSDP's `FlatParameter` as "flat parameter", not "flattened parameter". This PR renames that in `flat_param.py`.
**This PR only changes documentation.**
Pull Request resolved: https://github.com/pytorch/pytorch/pull/97661
Approved by: https://github.com/rohan-varma