code: def dKd_dVar(self, X, X2=None):
  		
  		r = self._scaled_dist(X, X2)
  		return self.dKdVar_of_r(r)
docs: Derivative of Kernel function wrt variance applied on inputs X and X2.
  		In the stationary case there is an inner function depending on the
  		distances from X to X2, called r.
  
  		dKd_dVar(X, X2) = dKdVar_of_r((X-X2)**2)