Remove global group name tracking for ProcessGroupNCCL (#25905)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25905
Now that we can detect and recover from failures in NCCL we should
allow processes that are started at different times (and perhaps have
had previous NCCL process group instances), to eventually be part of
the same process group. Keeping track of group names in global
variables prevents that, because the processes will be out of sync.
This commit removes the global group name maps and defers
responsibility of isolating access to the same store from multiple
process groups to the store itself. Users can use `c10d::PrefixStore`
to derive new store instances whose keyspace is scoped to some
prefix. Functionally, this is identical to keeping a global map and
using a group name, but also gives more flexibility to the front-end
API to reset state and have processes that have started at different
times to join the same process group.
ghstack-source-id: 89804865
Test Plan: Tests pass.
Differential Revision: D17281416
fbshipit-source-id: eab3b48463a9b0ef24aedeca76e2bb970b9f33ef
