Enforce explicit ProcessGroup passed into DefaultState (#84105)
Would prefer to enforce that users pass in explicit PG into these state objects when using comm hooks with FSDP, so that it is clear and easy debugable over which processes communication is taking place.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/84105
Approved by: https://github.com/mrshenli, https://github.com/zhaojuanmao
