message: In ring_reducer.cc if the reduction does not need a final op, then
  signal group_size_tensor_ready_ immediately, without initialization.
diff: @@ -206,6 +206,9 @@ void RingReducer::ContinueAfterInputCopy() {
  group_size_tensor_ = group_size_val;
  group_size_tensor_ready_.Notify();
  }
  + } else {
  + // Value won't be used, so no need to initialize.
  + group_size_tensor_ready_.Notify();
  }
  Finish(RunAsyncParts());
  }
  