docstr: Helper function, used to make FFT model pickable.
docstr_tok: ["Helper","function",",","used","to","make","FFT","model","pickable","."]
code: def _exp_trend(self, x) -> Callable:
      """Helper function, used to make FFT model pickable."""
      return np.exp(self.trend_coefficients[1]) * np.exp(
          self.trend_coefficients[0] * x
      )
  
imports: from typing import Callable, Optional
  import numpy
  import pandas
  from statsmodels.tsa.stattools import acf
  from darts.logging import get_logger
  from darts.models.forecasting.forecasting_model import LocalForecastingModel
  from darts.timeseries import TimeSeries
  from darts.utils.missing_values import fill_missing_values
label_desc: Use image node_id 3 for calling the FFT obj's underlying member method code with example usage: obj._exp_trend(x) and returns: unknown