code: def test_forward_cpu(self):
      with chainer.using_config("use_cudnn", "always"):
          self.check_forward(self.x, self.t)
  
imports: import math
  import unittest
  import numpy
  import six
  import chainer
  from chainer.backends import cuda
  from chainer import functions
  from chainer import gradient_check
  from chainer import testing
  from chainer.testing import attr
  from chainer import utils
label_desc: Use image node_id 4 for calling the TestSigmoidCrossEntropy obj's underlying member method code with example usage: obj.test_forward_cpu() without return types