// live until `Join` finishes. If `is_async=true` it must live until `Join` is
  // followed by `TFE_ContextAsyncWait` to clear pending operations. It will be
  // used to cancel all other operations if any fails.
  //
  // Set step_id to configure the step id used for rendezvous creation. step id
  // of value -1 is reserved for global rendezvous and should not be set here.
  void StartExecute(TFE_Context* context,

        grad_it->second.push_back(grad);
      }
      result[i] = grad_it->second[0];
      used_gradient_ids.insert(tensor_id);
    }
  }
  VLOG(1) << "Final gradients size: "
          << gradients.size() - used_gradient_ids.size();
  for (const auto& grad_pair : gradients) {
    if (used_gradient_ids.find(grad_pair.first) == used_gradient_ids.end()) {
      for (const auto& g : grad_pair.second) {
        vspace.DeleteGradient(g);