[chat]Move the function "save_model" into PPOTrainer

[Dr-Corgi]

The function save_model should be a part of PPOTrainer.

save_model函数被错误地外置到模型外部，导致rlhf train阶段无法保存模型。

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[ht-zhou]

Thanks for your nice work. The bug occurred because some code changes occurred after our training, in order to adapt to the pipeline of huggingface.