code: def backward_impl(self, inputs, outputs, propagate_down, accum):
      pass
  
imports: import sys
  import pytest
  import numpy
  import nnabla
  import nnabla.functions
  import nnabla.parametric_functions
  import nnabla.initializer
  from nnabla.ext_utils import get_extension_context
  from nbla_test_utils import list_context
  from nnabla.testing import assert_allclose
  from nnabla.function import PythonFunction
  from nnabla.backward_functions import registry, register
label_desc: Use image node_id 7 for calling the IdentityForwardOnlyFunction obj's underlying member method code with example usage: obj.backward_impl(inputs, outputs, propagate_down, accum) without return types