explicit ForwardFunction(lambda_type lambda)
      : std::function<Status(const std::vector<Gradient*>&,
                             std::vector<Gradient*>*, bool)>(lambda) {}
};

// Computes Jacobian-vector products using forward-mode automatic
// differentiation.
//
// While GradientTape's RecordOperation is trivial, ForwardAccumulator's
// Accumulate runs the gradient computation immediately.
//