class_docstr: Implementation of Radial Basis Function kernel:
  
      :math:`k(x,z) = \sigma^2\exp\left(-0.5 \times \frac{|x-z|^2}{l^2}\right).`
  
  .. note:: This kernel also has name `Squared Exponential` in literature.
code: def forward(self, X, Z=None, diag=False):
      if diag:
          return self._diag(X)
  
      r2 = self._square_scaled_dist(X, Z)
      return self.variance * torch.exp(-0.5 * r2)
  
imports: import torch
  from torch.distributions import constraints
  from pyro.contrib.gp.kernels.kernel import Kernel
  from pyro.nn.module import PyroParam
label_desc: Use image node_id 2 for calling the RBF obj's underlying member method code with example usage: obj.forward(X, Z, diag) and returns: unknown, self