code: def save(self, checkpoint_file):
      from bigdl.nano.utils.common import invalidInputError
  
      if self.model is None:
          invalidInputError(
              False,
              "Needs to call fit_eval or restore first before calling save",
          )
      with open(checkpoint_file, "wb") as fout:
          pickle.dump(self.model, fout)
  
file_path: BigDL/python/chronos/src/bigdl/chronos/model/arima.py
imports: import os
  import pickle
  from pmdarima.arima import ARIMA
  from pmdarima.arima import ndiffs
  from pmdarima.arima import nsdiffs
  from bigdl.chronos.metric.forecast_metrics import Evaluator
label_desc: Use image node_id 6 for calling the ARIMAModel obj's underlying member method code with example usage: obj.save(checkpoint_file) without return types