code: def test_invalid_double_backprop(self):
      with self.assertRaises(RuntimeError):
          x = functions.forget(lambda v: v, self.v)
          x.grad_var = variable.Variable(numpy.ones_like(x.data))
          x.backward(enable_double_backprop=True)
  
imports: import functools
  import unittest
  import numpy
  import six
  import chainer
  from chainer import cuda
  from chainer import functions
  from chainer import gradient_check
  from chainer import links
  from chainer import testing
  from chainer.testing import attr
  from chainer import variable
label_desc: Use image node_id 2 for calling the TestForgetDoubleBackpropError obj's underlying member method code with example usage: obj.test_invalid_double_backprop() without return types