code: def test_double_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)
      ggx = src_backend_config.get_array(self.ggx)
  
      x_var = chainer.Variable(x, requires_grad=True)
  
      y_var = functions.copy(x_var, dst_device)
  
      y_var.grad = gy
  
      gy_var = y_var.grad_var
      y_var.backward(enable_double_backprop=True)
  
      assert x_var.grad_var.requires_grad is True
  
      x_var.grad_var.grad = ggx
      x_var.grad_var.backward()
  
      assert gy_var.grad_var.device == dst_device
      assert (
          backend.get_device_from_array(gy_var.grad_var.array)
          == dst_device
      )
      numpy.testing.assert_array_equal(
          _numpy_device.send(gy_var.grad_var.array), self.ggx
      )
  
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 6 for calling the CopyTestBase obj's underlying member method code with example usage: obj.test_double_backward(src_backend_config, dst_backend_config) without return types