code: def test_edge_nan():
      img = np.zeros((10, 10))
      img[:5] = 1
      img[0] = np.nan
      grad_mag, _ = _edge_detect(img)
      np.testing.assert_almost_equal(img[4], 1)
      assert (grad_mag[0] > 2).all()
  
imports: import numpy
  from nilearn.plotting.edge_detect import _edge_detect
label_desc: Use image node_id 2 for calling a global function with example usage: test_edge_nan() without return types