code: def backward(self, indexes, gy):
      z = self.get_retained_inputs()[0]
      xp = backend.get_array_module(*gy)
      return (chainer.functions.polygamma(xp.array(1), z) * gy[0],)
  
file_path: chainer/chainer/functions/math/digamma.py
imports: import chainer
  from chainer import backend
  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 DiGamma obj's underlying member method code with example usage: obj.backward(indexes, gy) and returns: unknown