code: def backward(self, indexes, gy):
      n, x = self.get_retained_inputs()
      return (
          None,
          polygamma(n + 1, x) * gy[0],
      )
  
file_path: chainer/chainer/functions/math/polygamma.py
imports: from chainer.backends import cuda
  from chainer import function_node
  from chainer import utils
  from chainer.utils import type_check
label_desc: Use image node_id 5 for calling the PolyGamma obj's underlying member method code with example usage: obj.backward(indexes, gy) and returns: None,  unknown