[DDP] Remove the outdated limitations of DDP communication hook since 1.9 (#8346)
[DDP] Remove the outdated limitations of DDP communication hook since 1.9 (#8346)

* [DDP] Remove the outdated limitations of DDP communication hook since 1.9

1. DDP communication hook can work on multiple backends since 1.9.
2. SPMD in DDP is completely retired in 1.9, and SPSD is the only option.

* Update ddp.py

Co-authored-by: pre-commit-ci[bot] <3ea08453efb076ca910552532a53973a6527df40@users.noreply.github.com>
williamFalcon/pytorch-lightning,williamFalcon/pytorch-lightning