Returns the kernel evaluated on `r2`, which is the scaled squared distance.
        Will call self.K_r(r=sqrt(r2)), or can be overwritten directly (and should operate element-wise on r2).

def K_r2(self, r2):
        """
        Returns the kernel evaluated on `r2`, which is the scaled squared distance.
        Will call self.K_r(r=sqrt(r2)), or can be overwritten directly (and should operate element-wise on r2).
        """
        r = self._clipped_sqrt(r2)
        return self.K_r(r)