code: def test_backward_simple(self):
      self._test_replace_func(
          lambda a, b: a * b,
          [
              chainer.Variable(np.array(2.3)),
              chainer.Variable(np.array(4.2)),
          ],
      )
  
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 TestReplaceFuncBackward obj's underlying member method code with example usage: obj.test_backward_simple() without return types