message: Fix reduce op ut failure: root rank=-1
diff: @@ -452,6 +452,7 @@ void NcclManager::AddReduceSend(std::unique_ptr<Participant> participant,
  void NcclManager::AddReduceRecv(std::unique_ptr<Participant> participant,
  const Context& context,
  ncclRedOp_t reduction_op) {
  + participant->root = true;
  AddParticipant(std::move(participant), context, kReduce, reduction_op);
  }
  