[chat] refactor model save/load logic

[ver217]

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1. Refactor unwrap_xxx method of strategy. Add a function get_base_model() to get base model. For Actor, the base model is the original lm model. For Critic and Reward model, the base model is itself. Strategy.unwrap_model is to get original lm model.
2. Refactor model save logic. Strategy.save_model always save in pytorch format. And Strategy.save_pretrained will save in HF format. However, ColossalAI strategy with stage-3 doesn't support save_pretrained.
3. Update examples and readme.

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