code: def save(self, prefix, epoch=None):
          
          if epoch is None:
              epoch = self.num_epoch
          assert epoch is not None
          save_checkpoint(prefix, epoch, self.symbol, self.arg_params, self.aux_params)
docs: Checkpoint the model checkpoint into file.
          You can also use `pickle` to do the job if you only work on Python.
          The advantage of `load` and `save` (as compared to `pickle`) is that
          the resulting file can be loaded from other MXNet language bindings.
          One can also directly `load`/`save` from/to cloud storage(S3, HDFS)
  
          Parameters
          ----------
          prefix : str
              Prefix of model name.
  
          Notes
          -----
          - ``prefix-symbol.json`` will be saved for symbol.
          - ``prefix-epoch.params`` will be saved for parameters.