add 'node_rank' to torchrun cmd (pytorch#751)

* add 'node_rank' to torchrun cmd

* add comment

Author: MichaelClifford

torchx/components/dist.py

@@ -262,6 +262,9 @@ def ddp(
         nnodes_rep,
         "--nproc_per_node",
         str(nproc_per_node),
+        # node_rank is only used when rdzv_backend is 'static'
+        "--node_rank",
+        f"{macros.replica_id}",
         "--tee",
         "3",
         "--role",