code: def backward_impl(self, inputs, outputs, propagate_down, accum):
      if propagate_down[0]:
          if accum[0]:
              inputs[0].g += outputs[0].g
          else:
              inputs[0].g = outputs[0].g
      if propagate_down[1]:
          if accum[1]:
              inputs[1].g += outputs[0].g
          else:
              inputs[1].g = outputs[0].g
  
file_path: nnabla/python/test/test_python_function.py
imports: import pytest
  import numpy
  import nnabla.functions
  import nnabla
  from nnabla.function import PythonFunction
label_desc: Use image node_id 8 for calling the Add2 obj's underlying member method code with example usage: obj.backward_impl(inputs, outputs, propagate_down, accum) without return types