const std::unordered_map<int64_t, int64_t>& op_missing_tensor) {
  std::vector<int64_t> result;
  for (auto& op_entry : op_tape) {
    if (op_missing_tensor.find(op_entry.first) == op_missing_tensor.end()) {
      result.push_back(op_entry.first);
    }
  }
  return result;
}

template <typename Gradient, typename BackwardFunction, typename TapeTensor>
Status InitialGradients(

//       .Output("output: T")
//       .Attr("N: int >= 2")
//       .Attr("T: type");
// that defines two inputs, "concat_dim" and "values" (in that order).
// You must use TF_AddInput() for the first input (since it takes a
// single tensor), and TF_AddInputList() for the second input (since
// it takes a list, even if you were to pass a list with a single
// tensor), as in: