[FSDP][1/N] Retire `FlattenParamsWrapper` (#86117)
This deprecates `FlattenParamsWrapper`'s usage for "unflattening" the original parameters. After this PR, FPW only serves to register and de-register its `FlatParameter` for the parent `FullyShardedDataParallel` instance.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/86117
Approved by: https://github.com/zhaojuanmao
