Saves the trial's state to a checkpoint.

def save(self, trial, storage=Checkpoint.DISK):
        """Saves the trial's state to a checkpoint."""
        trial._checkpoint.storage = storage
        trial._checkpoint.last_result = trial.last_result
        if storage == Checkpoint.MEMORY:
            trial._checkpoint.value = trial.runner.save_to_object.remote()
        else:
            # Keeps only highest performing checkpoints if enabled
            if trial.keep_checkpoints_num:
                try:
                    last_attr_val = trial.last_result[
                        trial.checkpoint_score_attr]
                    if (trial.compare_checkpoints(last_attr_val)
                            and not math.isnan(last_attr_val)):
                        trial.best_checkpoint_attr_value = last_attr_val
                        self._checkpoint_and_erase(trial)
                except KeyError:
                    logger.warning(
                        "Result dict has no key: {}. keep"
                        "_checkpoints_num flag will not work".format(
                            trial.checkpoint_score_attr))
            else:
                with warn_if_slow("save_to_disk"):
                    trial._checkpoint.value = ray.get(
                        trial.runner.save.remote())

        return trial._checkpoint.value