Forward mode differentiation for a constant

def _forward_mode(self, *args):
        """Forward mode differentiation for a constant"""
        # Evaluate inner function self.f
        X: np.ndarray
        dX: np.ndarray
        X, dX = self.f._forward_mode(*args)
        # The function value
        val = self.func(X)
        # The derivative
        diff = self.deriv(X) * dX
        return (val, diff)