code: def backward(self, indexes, gy):
      (y,) = self.get_retained_outputs()
      sqrt_2pi = (2 * math.pi) ** 0.5
      return (sqrt_2pi * chainer.functions.exp(0.5 * y**2) * gy[0],)
  
file_path: chainer/chainer/functions/math/ndtri.py
imports: import math
  import chainer
  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 Ndtri obj's underlying member method code with example usage: obj.backward(indexes, gy) and returns: unknown