class_docstr: Base class for a family of isotropic covariance kernels which are functions of the
  distance :math:`|x-z|/l`, where :math:`l` is the length-scale parameter.
  
  By default, the parameter ``lengthscale`` has size 1. To use the isotropic version
  (different lengthscale for each dimension), make sure that ``lengthscale`` has size
  equal to ``input_dim``.
  
  :param torch.Tensor lengthscale: Length-scale parameter of this kernel.
docstr: Calculates the diagonal part of covariance matrix on active features.
docstr_tok: ["Calculates","the","diagonal","part","of","covariance","matrix","on","active","features","."]
code: def _diag(self, X):
      """
      Calculates the diagonal part of covariance matrix on active features.
      """
      return self.variance.expand(X.size(0))
  
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 4 for calling the Isotropy obj's underlying member method code with example usage: obj._diag(X) and returns: self