code: def _test_function(x, a, b):
      return a * x + b
  
file_path: sktime/sktime/forecasting/trend/_curve_fit_forecaster.py
imports: import pandas
  from scipy.optimize import curve_fit
  from sktime.forecasting.base import BaseForecaster
  from sktime.forecasting.trend._util import _get_X_numpy_int_from_pandas
label_desc: Use image node_id 1 for calling a global function with example usage: _test_function(x, a, b) and returns: unknown