class_docstr: Implementation of Matern52 kernel:
  
      :math:`k(x,z)=\sigma^2\left(1+\sqrt{5}\times\frac{|x-z|}{l}+\frac{5}{3}\times
      \frac{|x-z|^2}{l^2}\right)\exp\left(-\sqrt{5} \times \frac{|x-z|}{l}\right).`
code: def forward(self, X, Z=None, diag=False):
      if diag:
          return self._diag(X)
  
      r2 = self._square_scaled_dist(X, Z)
      r = _torch_sqrt(r2)
      sqrt5_r = 5**0.5 * r
      return (
          self.variance
          * (1 + sqrt5_r + (5 / 3) * r2)
          * torch.exp(-sqrt5_r)
      )
  
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 Matern52 obj's underlying member method code with example usage: obj.forward(X, Z, diag) and returns: unknown, self