code: def test_hook_for_funcnode(self, test_type):
      class Model(chainer.Chain):
          def forward(self, x):
              if test_type in ["variable", "array"]:
                  x = [chainer.as_variable(x)]
              elif test_type == "dict":
                  x = list(x.values())
              x.append(chainer.Variable(np.array(7, np.float32)))
              return F.stack(x)
  
      model = Model()
      x = self.get_x(test_type)
      with RetainInputHook() as h:
          model(x)
      expected_count = 1
      if test_type == "array":
          # input is ndarray and not checked in forward_preprocess
          expected_count += 1
      assert len(h.retain_inputs) == expected_count
  
imports: import unittest
  import chainer
  import chainer.functions
  import chainer.links
  from chainer import testing
  import numpy
  import pytest
  from onnx_chainer import export
  from onnx_chainer.export import RetainInputHook
  from onnx_chainer.testing import input_generator
  from onnx_chainer_tests.helper import ONNXModelTest
label_desc: Use image node_id 2 for calling the TestRetainInputHook obj's underlying member method code with example usage: obj.test_hook_for_funcnode(test_type) and returns: F