code: def test_TimeSeriesDifferentiator_fit_initial_values():
      """
      Test that TimeSeriesDifferentiator fit method sets initial_values attribute.
      """
      X = np.arange(10)
      tsd = TimeSeriesDifferentiator(order=1)
      tsd.fit(X)
  
      assert tsd.initial_values == [0]
      assert tsd.last_values == [9]
  
imports: import re
  import pytest
  import numpy
  from skforecast.preprocessing import TimeSeriesDifferentiator
label_desc: Use image node_id 2 for calling a global function with example usage: test_TimeSeriesDifferentiator_fit_initial_values() without return types