code: def test_dropout_backward(self):
      rspace, y = self.states.forward(None, self.x, self.ratio)
      gx = self.states.backward(None, self.gy, self.ratio, rspace)
  
      forward_mask = y / self.x
      backward_mask = gx / self.gy
  
      # backward_mask must be the same as forward_mask
      assert cupy.all(forward_mask == backward_mask)
  
imports: import sys
  import numpy
  import pytest
  import cupy
  import cupy.cuda.cudnn
  from cupy import testing
label_desc: Use image node_id 3 for calling the TestCudnnDropout obj's underlying member method code with example usage: obj.test_dropout_backward() without return types