[Reducer] small fix (pytorch#74127)

Summary:
Pull Request resolved: pytorch#74127

getRank() is now public.
ghstack-source-id: 151319258

Test Plan: CI

Reviewed By: zhaojuanmao

Differential Revision: D34833222

fbshipit-source-id: a846509e2d61dd9208706d471c7c83819ce24922
(cherry picked from commit 119974a)

torch/csrc/distributed/c10d/reducer.cpp

@@ -2126,10 +2126,9 @@ void verify_params_across_processes(
   i = 0;
   for (const auto p : c10::irange(params.size())) {
     const auto& t = params[p];
-    // I'd like to include which process we are in the message,
-    // but ProcessGroup::getRank is not public!
     for (const auto& sz : t.sizes()) {
-      auto msg = c10::str("params[", p, "] in this process",
+      auto msg = c10::str("[", process_group->getRank(),
+                        "]: params[", p, "] in this process",
                         " with sizes ",
                         t.sizes(),
                         " appears not to match sizes of the same param in process 0.");