code: def test_variable_set():
      # Testing hash and equality operator via set
      shape = (2, 3, 4)
      x = nn.Variable(shape)
      s = set()
      s.add(x)
      assert x in s
  
      y = nn.Variable(shape)
      assert y not in s
  
      y = x.get_unlinked_variable()
      assert y in s
  
      y.need_grad = True
      assert y in s
  
imports: import pytest
  import numpy
  import nnabla
  from nnabla.testing import assert_allclose
label_desc: Use image node_id 14 for calling a global function with example usage: test_variable_set() without return types