set ranks when initializing pg (meta-pytorch#280)

Summary:
Pull Request resolved: meta-pytorch#280

set metadata about ranks on the pg so that it can be used by flight recorder

Reviewed By: d4l3k

Differential Revision: D84101242

fbshipit-source-id: f5834e2db74f8f65dc3278f71aa6939f1e773784

Author: tushar00jain

torchft/process_group.py

@@ -791,6 +791,7 @@ def _create_pg(self, store: Store, rank: int, world_size: int) -> BaseProcessGro
         # pyre-fixme[16]: no attribute ProcessGroupNCCL
         opts = BaseProcessGroupNCCL.Options()
         opts.config.blocking = False
+        opts.global_ranks_in_group = list(range(world_size))
 
         pg = BaseProcessGroup(store, rank, world_size)
         pg._set_default_backend(ProcessGroup.BackendType.NCCL)