code: def test_forward_mat(self):
      test_x = torch.randn(4, 3)
      mean = self.create_mean()
      self.assertEqual(mean(test_x).shape, torch.Size([4, 3]))
      self.assertEqual(mean(test_x)[..., 1:].norm().item(), 0)
  
imports: import unittest
  import torch
  from gpytorch.means import ConstantMean, MultitaskMean, ZeroMean
  from gpytorch.test.base_mean_test_case import BaseMeanTestCase
label_desc: Use image node_id 3 for calling the TestMultitaskMean obj's underlying member method code with example usage: obj.test_forward_mat() without return types