// available in language bindings (e.g. Python).
  const VSpace<Gradient, BackwardFunction, TapeTensor>& vspace_;

  // Decides if tangents are vectorized or not
  bool use_batch_;

  struct AccumulatorCallState {
    AccumulatorCallState(
        GradientTape<Gradient, BackwardFunction, TapeTensor>* backward_tape,
        bool accumulating)
        : backward_tape(backward_tape), accumulating(accumulating) {}