code: def test_grad_error(self):
      model = self.get_model()
      # this alternative function does not return chainer.Variable
      # backward propagation will fail
      model.half = fake_as_funcnode(
          lambda xs, value=0.5: xs.array * value, "MulConstant"
      )
      x = input_generator.increasing(2, 5)
  
      with pytest.raises(ValueError):
          self.expect(model, x, output_grad=True)
  
imports: import os
  import warnings
  import chainer
  import chainer.functions
  import chainer.links
  from chainer import testing
  import numpy
  import onnx
  import pytest
  from onnx_chainer import export
  from onnx_chainer import export_testcase
  from onnx_chainer import onnx_helper
  from onnx_chainer.replace_func import as_funcnode
  from onnx_chainer.replace_func import fake_as_funcnode
  from onnx_chainer.testing import input_generator
  from onnx_chainer_tests.helper import ONNXModelChecker
  from onnx_chainer_tests.helper import ONNXModelTest
label_desc: Use image node_id 2 for calling the TestReplaceWithOutputGrad obj's underlying member method code with example usage: obj.test_grad_error() without return types