class_docstr: Implementation of Cosine kernel:
  
      :math:`k(x,z) = \sigma^2 \cos\left(\frac{|x-z|}{l}\right).`
  
  :param torch.Tensor lengthscale: Length-scale parameter of this kernel.
code: def forward(self, X, Z=None, diag=False):
      if diag:
          return self._diag(X)
  
      r = self._scaled_dist(X, Z)
      return self.variance * torch.cos(r)
  
file_path: pyro/pyro/contrib/gp/kernels/periodic.py
imports: import math
  import torch
  from torch.distributions import constraints
  from pyro.contrib.gp.kernels.isotropic import Isotropy
  from pyro.contrib.gp.kernels.kernel import Kernel
  from pyro.nn.module import PyroParam
label_desc: Use image node_id 2 for calling the Cosine obj's underlying member method code with example usage: obj.forward(X, Z, diag) and returns: unknown, self