code: def test_num_classes_check():
      X, y = get_x_y(n_samples=1000, n_classes=1)
      kf = StratifiedKFold(n_splits=5)
      err_msg = "number of unique classes cannot be less than 2"
      with pytest.raises(ValueError, match=err_msg):
          for train_index, test_index in kf.split(X, y):
              pass
  
imports: from cuml.model_selection import StratifiedKFold
  import pytest
  from cuml.internals.safe_imports import gpu_only_import
label_desc: Use image node_id 3 for calling a global function with example usage: test_num_classes_check() without return types