code: def test_forecaster_index_step_stored():
      """
      Test serie without DatetimeIndex, step is stored in forecaster.index_freq.
      """
      series = pd.DataFrame(
          {"1": pd.Series(np.arange(5)), "2": pd.Series(np.arange(5))}
      )
  
      forecaster = ForecasterAutoregMultiSeriesCustom(
          regressor=LinearRegression(),
          fun_predictors=create_predictors,
          window_size=3,
      )
      forecaster.fit(series=series)
      expected = series.index.step
      results = forecaster.index_freq
  
      assert results == expected
  
file_path: skforecast/skforecast/ForecasterAutoregMultiSeriesCustom/tests/test_fit.py
imports: import re
  import pytest
  import numpy
  import pandas
  from skforecast.ForecasterAutoregMultiSeriesCustom import ForecasterAutoregMultiSeriesCustom
  from sklearn.linear_model import LinearRegression
  from sklearn.preprocessing import StandardScaler
label_desc: Use image node_id 5 for calling a global function with example usage: test_forecaster_index_step_stored() without return types