code: def test_is_stationary(self):
      kernel_1 = RBFKernel().initialize(lengthscale=1)
      kernel_2 = RBFKernel().initialize(lengthscale=2)
      kernel_3 = LinearKernel().initialize()
  
      self.assertTrue((kernel_1 + kernel_2).is_stationary)
      self.assertTrue((kernel_1 * kernel_2).is_stationary)
      self.assertFalse((kernel_1 + kernel_3).is_stationary)
      self.assertFalse((kernel_1 * kernel_3).is_stationary)
  
imports: import math
  import unittest
  import linear_operator
  import torch
  import gpytorch
  from gpytorch.kernels import LinearKernel, MaternKernel, RBFKernel, RFFKernel
label_desc: Use image node_id 14 for calling the TestAdditiveAndProductKernel obj's underlying member method code with example usage: obj.test_is_stationary() without return types