code: def test_warn_no_grad(self):
      x = chainer.Variable(np.array(4, np.float32))
      x.grad = np.array(3, np.float32)
      y = x * 2
      with testing.assert_warns(RuntimeWarning):
          chainer.backward([y])
      testing.assert_allclose(x.grad, np.array(3, np.float32))
      assert y.grad is None
  
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 10 for calling the TestBackward obj's underlying member method code with example usage: obj.test_warn_no_grad() without return types