std::vector<tensorflow::Output> dy_arg;

  {
    // We need to hold on to the lock while we have a scope that uses TF_Graph.
    mutex_lock graph_lock(g->mu);

    const int first_new_node_id = g->graph.num_node_ids();

    string prefix_cmp;
    const char* child_scope_name;
    if (prefix == nullptr) {
      child_scope_name = "gradients";
    } else {
      prefix_cmp = string(prefix) + "/";