code: def test_kissgp_gp_mean_abs_error(self, toeplitz=False):
      likelihood = GaussianLikelihood()
      gp_model = GPRegressionModel(train_x, train_y, likelihood)
      mll = gpytorch.mlls.ExactMarginalLogLikelihood(
          likelihood, gp_model
      )
  
      with gpytorch.settings.max_preconditioner_size(
          10
      ), gpytorch.settings.max_cg_iterations(50):
          with gpytorch.settings.fast_pred_var(), gpytorch.settings.use_toeplitz(
              toeplitz
          ):
              # Optimize the model
              gp_model.train()
              likelihood.train()
  
              optimizer = optim.Adam(gp_model.parameters(), lr=0.01)
              optimizer.n_iter = 0
              for _ in range(15):
                  optimizer.zero_grad()
                  output = gp_model(train_x)
                  loss = -mll(output, train_y)
                  loss.backward()
                  optimizer.n_iter += 1
                  optimizer.step()
  
                  for param in gp_model.parameters():
                      self.assertTrue(param.grad is not None)
                      self.assertGreater(param.grad.norm().item(), 0)
  
              # Test the model
              gp_model.eval()
              likelihood.eval()
  
              test_preds = likelihood(gp_model(test_x)).mean
              mean_abs_error = torch.mean(
                  torch.abs(test_y - test_preds)
              )
              self.assertLess(mean_abs_error.squeeze().item(), 0.2)
  
file_path: gpytorch/test/examples/test_kissgp_additive_regression.py
imports: import os
  import random
  import unittest
  import torch
  from torch import optim
  import gpytorch
  from gpytorch.distributions import MultivariateNormal
  from gpytorch.kernels import AdditiveStructureKernel, GridInterpolationKernel, RBFKernel, ScaleKernel
  from gpytorch.likelihoods import GaussianLikelihood
  from gpytorch.means import ZeroMean
label_desc: Use image node_id 3 for calling the TestKISSGPAdditiveRegression obj's underlying member method code with example usage: obj.test_kissgp_gp_mean_abs_error(toeplitz) without return types