code: def backward(self, indexes, gy):
      return (gy[0],)
  
file_path: chainer/tests/chainer_tests/testing_tests/test_unary_math_function_test.py
imports: import unittest
  from chainer import function_node
  from chainer import testing
label_desc: Use image node_id 3 for calling the DummyLinear obj's underlying member method code with example usage: obj.backward(indexes, gy) and returns: gy