class_docstr: Implementation of Exponential kernel:
  
      :math:`k(x, z) = \sigma^2\exp\left(-\frac{|x-z|}{l}\right).`
code: def __init__(
      self, input_dim, variance=None, lengthscale=None, active_dims=None
  ):
      super().__init__(input_dim, variance, lengthscale, active_dims)
  
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 1 to create a new Exponential object from inherited base classes: Isotropy with example: obj = Exponential(input_dim, variance, lengthscale, active_dims)