VLOG(1) << "Tensor " << id << " is source";
        continue;
      }
      auto missing_it = state.op_missing_tensor.find(op_id);
      if (missing_it != state.op_missing_tensor.end()) {
        missing_it->second--;
        VLOG(1) << "Op " << op_id << " missing " << missing_it->second
                << " output gradients";
        if (missing_it->second == 0) {
          op_stack.insert(op_stack.begin(), op_id);
        }