[release/2.8] Using c10d.barrier() in test_extra_cuda_context test in test_c10d_nccl.py (#2522)

Cherry-pick of #2447

Co-authored-by: akashveramd <Akash.Verma3@amd.com>

Author: dhonnappa-amd

test/distributed/test_c10d_nccl.py

@@ -639,6 +639,14 @@ def _helper_test_extra_cuda_context_by_memory(self):
         """
         device = torch.device(f"cuda:{self.rank:d}")
         x = torch.empty((1,), device=device)
+
+        # We need this barrier to ensure that all nodes have completed init_process_group
+        # If rank=0 gets a mem snapshot before other nodes have finished init_process_group,
+        # then we artificially see a bump in memory usage. As per the following comment,
+        # we are going to be moving away from this function:
+        # https://github.com/pytorch/pytorch/pull/154174#discussion_r2105065931
+        c10d.barrier()
+
         # Rank 0 takes a snapshot before collective -- this snapshot should have
         # included rank 0's own context.
         if self.rank == 0: