code: def ref_dropout_double_backward(dy, mask, p):
      """This function returns the reference result of the derivative of
      y + dy/dx where y = F.Dropout. Because the second derivative of Dropout
      is 0, this returns y' + 0.
      """
      return ref_dropout_backward(dy, mask, p)
  
file_path: nnabla/python/test/function/test_dropout.py
imports: from six.moves import range
  import pytest
  import numpy
  import nnabla
  import nnabla.functions
  from nbla_test_utils import list_context
  from nnabla.testing import assert_allclose
label_desc: Use image node_id 4 for calling a global function with example usage: ref_dropout_double_backward(dy, mask, p) and returns: ref_dropout_backward