code: def save_params(step_num, model, trainer, ckpt_dir):
      
      param_path = os.path.join(ckpt_dir, '%07d.params'%step_num)
      trainer_path = os.path.join(ckpt_dir, '%07d.states'%step_num)
      logging.info('[step %d] Saving checkpoints to %s, %s.',
                   step_num, param_path, trainer_path)
      model.save_parameters(param_path)
      trainer.save_states(trainer_path)
docs: Save the model parameter, marked by step_num.