code: @Override
    public void reduce(DeepLearningTask2 drt) {
      if (_res == null) _res = drt._res;
      else {
        _res._chunk_node_count += drt._res._chunk_node_count;
        _res.model_info().add(drt._res.model_info()); //add models, but don't average yet
      }
      assert(_res.model_info().get_params()._replicate_training_data);
    }
docs: Reduce between worker nodes, with network traffic (if greater than 1 nodes)
  After all reduce()'s are done, postGlobal() will be called
  @param drt task to reduce