code: def test_gray2d_default_channel_axis():
      img = np.zeros((20, 21))
      img[:10, :10] = 0.33
      with pytest.raises(
          ValueError, match="channel_axis=-1 indicates multichannel"
      ):
          slic(img)
      slic(img, channel_axis=None)
  
imports: from itertools import product
  import numpy
  import pytest
  from numpy.testing import assert_equal
  from skimage import data, filters, img_as_float
  from skimage._shared.testing import run_in_parallel, expected_warnings
  from skimage.segmentation import slic
label_desc: Use image node_id 4 for calling a global function with example usage: test_gray2d_default_channel_axis() without return types