code: def check_backward(self, x, gy):
      x = chainer.Variable(x)
      y = self.f.apply((x,))[0]
      y.grad = gy
      y.backward()
      self.assertEqual(2, len(self.h.call_history))
      check_history(
          self,
          self.h.call_history[1],
          functions.math.exponential.Exp,
          int,
          int,
      )
  
imports: import unittest
  import numpy
  import six
  import chainer
  from chainer.backends import cuda
  from chainer import function_hooks
  from chainer import functions
  from chainer.functions.math import basic_math
  from chainer import testing
  from chainer.testing import attr
label_desc: Use image node_id 5 for calling the TestCupyMemoryProfileHookToFunction obj's underlying member method code with example usage: obj.check_backward(x, gy) without return types