[FSDP][5/N] Update `FlatParamHandle` after FPW deprecation (#87113)
This PR resolves a TODO left in `FlatParamHandle` that was conditional on deprecating `FlattenParamsWrapper`. We simply pass in the process group into the `FlatParamHandle` constructor instead of later in `shard()`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/87113
Approved by: https://github.com/zhaojuanmao
