code: def test_backward_cpu(self):
      self.check_backward(self.x, self.rois, self.roi_indices, self.gy)
  
imports: import chainer
  from chainer.backends import cuda
  from chainer import gradient_check
  from chainer import testing
  from chainer.testing import attr
  from chainer.testing import condition
  import numpy
  import unittest
  from chainercv import functions
  from tests.functions_tests.test_ps_roi_average_pooling_2d import _outsize
label_desc: Use image node_id 6 for calling the TestPSROIAverageAlign2D obj's underlying member method code with example usage: obj.test_backward_cpu() without return types