docstr: Tests that `Diff` updates `start_vals` parameter when refitted to new data.
docstr_tok: ["Tests","that","`","Diff","`","updates","`","start_vals","`","parameter","when","refitted","to","new","data","."]
code: def test_diff_multiple_calls_to_fit(self):
      """
      Tests that `Diff` updates `start_vals` parameter when refitted to new data.
      """
      diff = Diff(lags=[2, 2], dropna=True)
  
      diff.fit(self.sine_series)
      startvals1 = deepcopy(diff._fitted_params)[0][0]
  
      diff.fit(self.sine_series + 1)
      startvals2 = deepcopy(diff._fitted_params)[0][0]
  
      assert not np.allclose(startvals1, startvals2)
  
imports: from copy import deepcopy
  from typing import Optional, Sequence
  import numpy
  import pandas
  import pytest
  from darts.dataprocessing.transformers import Diff
  from darts.timeseries import TimeSeries
  from darts.timeseries import concatenate
  from darts.utils.timeseries_generation import linear_timeseries, sine_timeseries
label_desc: Use image node_id 11 for calling the TestDiff obj's underlying member method code with example usage: obj.test_diff_multiple_calls_to_fit() without return types