code: def _checkpoint_and_erase(self, trial):
          
  
          with warn_if_slow("save_to_disk"):
              trial._checkpoint.value = ray.get(trial.runner.save.remote())
  
          if len(trial.history) >= trial.keep_checkpoints_num:
              ray.get(trial.runner.delete_checkpoint.remote(trial.history[-1]))
              trial.history.pop()
  
          trial.history.insert(0, trial._checkpoint.value)
docs: Checkpoints the model and erases old checkpoints
              if needed.
          Parameters
          ----------
              trial : trial to save