code: def backward(self, indexes, gy):
      y = self.get_retained_outputs()[0]
      return (y * gy[0],)
  
imports: import math
  import numpy
  from chainer import backend
  from chainer.backends import cuda
  from chainer import function_node
  from chainer import utils
  from chainer.utils import type_check
  import chainerx
label_desc: Use image node_id 6 for calling the Exp obj's underlying member method code with example usage: obj.backward(indexes, gy) and returns: unknown