zero_indices.push_back(i);
          }
        }
      } else {
        any_gradient_nonzero = true;
        Gradient* new_gradients = nullptr;
        if (grad_it->second.size() == 1) {
          new_gradients = grad_it->second.at(0);
        } else {
          new_gradients = vspace.AggregateGradients(grad_it->second);
        }