[chat]fix save_model(#3377)

The function save_model should be a part of PPOTrainer.

applications/Chat/coati/trainer/ppo.py

@@ -116,6 +116,9 @@ class PPOTrainer(Trainer):
         self.critic_optim.zero_grad()
 
         return {'reward': experience.reward.mean().item()}
+    
+    def save_model(self, path: str, only_rank0: bool = False, tokenizer: Optional[PreTrainedTokenizerBase] = None) -> None:
+        self.strategy.save_model(model=self.actor, path=path, only_rank0=only_rank0, tokenizer=tokenizer)
 
     def save_model(self, path: str, only_rank0: bool = False, tokenizer: Optional[PreTrainedTokenizerBase] = None) -> None:
         self.strategy.save_model(model=self.actor, path=path, only_rank0=only_rank0, tokenizer=tokenizer)