code: def test_backward(self, src_backend_config, dst_backend_config):
      src_device = src_backend_config.device
      dst_device = dst_backend_config.device
      if (src_device.xp is chainerx) is not (dst_device.xp is chainerx):
          raise unittest.SkipTest(
              "ChainerX to non-ChainerX does not support backward."
          )
  
      x = src_backend_config.get_array(self.x)
      gy = dst_backend_config.get_array(self.gy)
  
      x_var = chainer.Variable(x, requires_grad=True)
  
      y_var = functions.copy(x_var, dst_device)
      y_var.grad = gy
  
      y_var.backward()
  
      x_grad = x_var.grad
      assert x_var.grad_var.device == src_device
      assert backend.get_device_from_array(x_grad) == src_device
      numpy.testing.assert_array_equal(
          _numpy_device.send(x_grad), self.gy
      )
  
imports: import unittest
  import numpy
  import pytest
  import chainer
  from chainer import backend
  from chainer.backends import _cpu
  from chainer.backends import cuda
  from chainer import functions
  from chainer import testing
  import chainerx
label_desc: Use image node_id 5 for calling the CopyTestBase obj's underlying member method code with example usage: obj.test_backward(src_backend_config, dst_backend_config) without return types