code: def test_initialize_levels_model_selection_multiseries_when_for_all_inputs(
      levels, levels_expected
  ):
      """
      Test initialize_levels_model_selection_multiseries when levels is None,
      str or list.
      """
      forecaster = ForecasterAutoregMultiSeries(
          regressor=Ridge(random_state=123), lags=2
      )
  
      levels = _initialize_levels_model_selection_multiseries(
          forecaster=forecaster, series=series, levels=levels
      )
  
      assert levels == levels_expected
  
file_path: skforecast/skforecast/model_selection_multiseries/tests/test_initialize_levels_model_selection_multiseries.py
imports: import re
  import pytest
  import numpy
  import pandas
  from sklearn.linear_model import Ridge
  from skforecast.ForecasterAutoregMultiSeries import ForecasterAutoregMultiSeries
  from skforecast.ForecasterAutoregMultiSeriesCustom import ForecasterAutoregMultiSeriesCustom
  from skforecast.ForecasterAutoregMultiVariate import ForecasterAutoregMultiVariate
  from skforecast.model_selection_multiseries.model_selection_multiseries import _initialize_levels_model_selection_multiseries
  from skforecast.exceptions import IgnoredArgumentWarning
  from .fixtures_model_selection_multiseries import series
label_desc: Use image node_id 4 for calling a global function with example usage: test_initialize_levels_model_selection_multiseries_when_for_all_inputs(levels, levels_expected) without return types