code: def fit(self, series: TimeSeries):
      super().fit(series)
  
      self.mean_val = np.mean(series.values(copy=False), axis=0)
      return self
  
imports: from typing import List, Optional, Sequence, Union
  import numpy
  from darts.logging import get_logger, raise_if, raise_if_not
  from darts.models.forecasting.ensemble_model import EnsembleModel
  from darts.models.forecasting.forecasting_model import ForecastingModel, LocalForecastingModel
  from darts.timeseries import TimeSeries
label_desc: Use image node_id 3 for calling the NaiveMean obj's underlying member method code with example usage: obj.fit(series) and returns: self