code: def save(self, checkpoint):
      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, "w") as fout:
          json.dump(model_to_json(self.model), fout)
  
file_path: BigDL/python/chronos/src/bigdl/chronos/model/prophet.py
imports: import json
  import numpy
  from prophet import Prophet
  from prophet.serialize import model_to_json, model_from_json
  from prophet.diagnostics import performance_metrics
  from prophet.diagnostics import cross_validation
  from bigdl.chronos.metric.forecast_metrics import Evaluator
label_desc: Use image node_id 8 for calling the ProphetModel obj's underlying member method code with example usage: obj.save(checkpoint) without return types