size = vspace.NumElements(unaggregated_grads[0]);
            gradients_size.emplace(id, size);
          } else {
            size = size_it->second;
          }
          if (unaggregated_grads.size() * size * 4 > kMinAggregateBytes) {
            Gradient* grad = vspace.AggregateGradients(unaggregated_grads);
            unaggregated_grads.clear();
            unaggregated_grads.push_back(grad);
          }
        }