code: def test_double_backward_floating_variables():
      x = nn.Variable((2, 2), need_grad=True)
      y = nn.Variable((2, 3), need_grad=True)
      z = nn.Variable((2, 4), need_grad=True)
      w = F.concatenate(*[x, y, z], axis=-1)
      o = F.sin(w)
      dx = nn.grad([o], [x])[0]
      ddx = nn.grad([dx], [x])[0]
  
imports: import sys
  import pytest
  import numpy
  import nnabla
  import nnabla.functions
  import nnabla.parametric_functions
  import nnabla.initializer
  from nnabla.ext_utils import get_extension_context
  from nbla_test_utils import list_context
  from nnabla.testing import assert_allclose
  from nnabla.function import PythonFunction
  from nnabla.backward_functions import registry, register
label_desc: Use image node_id 10 for calling a global function with example usage: test_double_backward_floating_variables() without return types