@Cache_this(limit=<NUM_LIT:3>, ignore_args=())<EOL><INDENT>def dKd_dVar(self, X, X2=None):<DEDENT>
r = self._scaled_dist(X, X2)<EOL>return self.dKdVar_of_r(r)<EOL>
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)