code: def test_backward(self):
      opt = Mock()
      loss = Mock()
      loss.backward = Mock()
  
      state = {
          torchbearer.X: None,
          torchbearer.MODEL: lambda x: None,
          torchbearer.Y_TRUE: None,
          torchbearer.CRITERION: lambda state: loss,
          torchbearer.LOSS: None,
          torchbearer.OPTIMIZER: opt,
          torchbearer.CALLBACK_LIST: MagicMock(),
          torchbearer.BACKWARD_ARGS: {},
      }
  
      closure = base_closure(
          torchbearer.X,
          torchbearer.MODEL,
          torchbearer.Y_PRED,
          torchbearer.Y_TRUE,
          torchbearer.CRITERION,
          torchbearer.LOSS,
          torchbearer.OPTIMIZER,
      )
      closure(state)
      self.assertTrue(loss.backward.call_count == 1)
  
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 12 for calling the TestBaseCrit obj's underlying member method code with example usage: obj.test_backward() without return types