code: def test_forecaster_index_step_stored():
      """
      Test serie without DatetimeIndex, step is stored in forecaster.index_freq.
      """
      series = pd.DataFrame(
          {
              "l1": pd.Series(np.arange(10)),
              "l2": pd.Series(np.arange(10)),
          }
      )
  
      forecaster = ForecasterAutoregMultiVariate(
          LinearRegression(), level="l1", lags=3, steps=2
      )
      forecaster.fit(series=series)
      expected = series.index.step
      results = forecaster.index_freq
  
      assert results == expected
  
file_path: skforecast/skforecast/ForecasterAutoregMultiVariate/tests/test_fit.py
imports: import re
  import pytest
  import numpy
  import pandas
  from skforecast.ForecasterAutoregMultiVariate import ForecasterAutoregMultiVariate
  from sklearn.linear_model import LinearRegression
  from sklearn.preprocessing import StandardScaler
label_desc: Use image node_id 4 for calling a global function with example usage: test_forecaster_index_step_stored() without return types