void ForwardAccumulator<Gradient, BackwardFunction, TapeTensor>::Watch(
    int64_t tensor_id, Gradient* tangent) {
  typename std::unordered_map<int64_t, Gradient*>::iterator existing =
      accumulated_gradients_.find(tensor_id);
  vspace_.MarkAsResult(tangent);
  if (existing == accumulated_gradients_.end()) {
    accumulated_gradients_.emplace(tensor_id, tangent);
  } else {