[device mesh][BE] remove allreduce from DM (#105605)
This PR removes allreduce from DM and use functional collective instead,
the rationle is that we don't want to maintain yet another set of
collective apis, and since the DM's collective is now a thin wrapper to functional collective so we
don't really need these collective to live in DM
Pull Request resolved: https://github.com/pytorch/pytorch/pull/105605
Approved by: https://github.com/kumpera, https://github.com/wz337, https://github.com/fduwjj
