int64_t tensor_id = output_tensors[i].GetID();
      auto existing = accumulated_gradients_.find(tensor_id);
      if (existing != accumulated_gradients_.end()) {
        // This is a somewhat odd case to be in, since it means we have two
        // operations which supposedly both created the same Tensor. It comes up
        // in recompute_grad, where the gradients have the same value. However,