Checkpoints the model and erases old checkpoints
            if needed.
        Parameters
        ----------
            trial : trial to save

def _checkpoint_and_erase(self, trial):
        """Checkpoints the model and erases old checkpoints
            if needed.
        Parameters
        ----------
            trial : trial to save
        """

        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)