code: def test_str(self):
      callback = Callback()
      self.assertEqual(
          str(callback).strip(), "torchbearer.bases.Callback"
      )
  
imports: import unittest
  from mock import Mock, MagicMock, create_autospec, patch
  import torch
  import torchbearer
  from torchbearer.bases import Metric, Callback, base_closure
  from torchbearer.metrics.primitives import CategoricalAccuracy
label_desc: Use image node_id 2 for calling the TestCallback obj's underlying member method code with example usage: obj.test_str() without return types