Gradient of the hessian (d2logpdf_dlink2) w.r.t variance parameter (t_noise)

        .. math::
            \\frac{d}{d\\sigma^{2}}(\\frac{d^{2} \\ln p(y_{i}|\lambda(f_{i}))}{d^{2}f}) = \\frac{v(v+1)(\\sigma^{2}v - 3(y_{i} - \lambda(f_{i}))^{2})}{(\\sigma^{2}v + (y_{i} - \lambda(f_{i}))^{2})^{3}}

        :param inv_link_f: latent variables link(f)
        :type inv_link_f: Nx1 array
        :param y: data
        :type y: Nx1 array
        :param Y_metadata: Y_metadata which is not used in student t distribution
        :returns: derivative of hessian evaluated at points f and f_j w.r.t variance parameter
        :rtype: Nx1 array

def d2logpdf_dlink2_dvar(self, inv_link_f, y, Y_metadata=None):
        """
        Gradient of the hessian (d2logpdf_dlink2) w.r.t variance parameter (t_noise)

        .. math::
            \\frac{d}{d\\sigma^{2}}(\\frac{d^{2} \\ln p(y_{i}|\lambda(f_{i}))}{d^{2}f}) = \\frac{v(v+1)(\\sigma^{2}v - 3(y_{i} - \lambda(f_{i}))^{2})}{(\\sigma^{2}v + (y_{i} - \lambda(f_{i}))^{2})^{3}}

        :param inv_link_f: latent variables link(f)
        :type inv_link_f: Nx1 array
        :param y: data
        :type y: Nx1 array
        :param Y_metadata: Y_metadata which is not used in student t distribution
        :returns: derivative of hessian evaluated at points f and f_j w.r.t variance parameter
        :rtype: Nx1 array
        """
        e = y - inv_link_f
        d2logpdf_dlink2_dvar = ( (self.v*(self.v+1)*(self.sigma2*self.v - 3*(e**2)))
                              / ((self.sigma2*self.v + (e**2))**3)
                           )
        return d2logpdf_dlink2_dvar