code: def forward(self, inputs, device):
      x, y = inputs
  
      mean = functions.mean(x, axis=1)
      d = x - mean[:, None]
      var = functions.mean(d * d, axis=1)
      inv_std = functions.rsqrt(var + self.eps)
  
      dummy_gamma = self.backend_config.xp.ones(
          self.shape[0], dtype=self.dtype
      )
  
      return gn_module._MulInvStd(
          self.eps, mean.array, inv_std.array, dummy_gamma
      ).apply((x, y))
  
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 2 for calling the TestMulInvStd obj's underlying member method code with example usage: obj.forward(inputs, device) and returns: gn_module