code: def forward_expected(self, inputs):
      x, y = inputs
      inv_std = (numpy.var(x, axis=1) + self.eps) ** -0.5
      z = inv_std[:, None] * y
      return (z,)
  
file_path: chainer/tests/chainer_tests/functions_tests/normalization_tests/test_group_normalization.py
imports: import numpy
  import six
  from chainer import functions
  import chainer.functions.normalization.group_normalization
  from chainer import testing
label_desc: Use image node_id 3 for calling the TestMulInvStd obj's underlying member method code with example usage: obj.forward_expected(inputs) and returns: z