code: def test_length_check(self):
      x = chainer.Variable(np.array(3, np.float32))
      y = chainer.functions.identity(x)
      gy = chainer.Variable(np.array(7, np.float32))
  
      with self.assertRaises(ValueError):
          chainer.backward([y], [])
      with self.assertRaises(ValueError):
          chainer.backward([y], [gy, gy])
      with self.assertRaises(ValueError):
          chainer.backward([], [gy])
      with self.assertRaises(ValueError):
          chainer.backward([y, y], [gy])
  
      chainer.backward([y], [gy])
  
imports: import unittest
  import mock
  import numpy
  import pytest
  import chainer
  from chainer.backends import cuda
  from chainer import testing
  from chainer.testing import attr
  import chainerx
label_desc: Use image node_id 1 for calling the TestBackwardValueCheck obj's underlying member method code with example usage: obj.test_length_check() without return types