[chat] refactor model save/load logic (#3654)

* [chat] strategy refactor unwrap model

* [chat] strategy refactor save model

* [chat] add docstr

* [chat] refactor trainer save model

* [chat] fix strategy typing

* [chat] refactor trainer save model

* [chat] update readme

* [chat] fix unit test

applications/Chat/coati/models/base/__init__.py

@@ -1,5 +1,24 @@
+import torch.nn as nn
+
 from .actor import Actor
 from .critic import Critic
 from .reward_model import RewardModel
 
-__all__ = ['Actor', 'Critic', 'RewardModel']
+
+def get_base_model(model: nn.Module) -> nn.Module:
+    """Get the base model of our wrapper classes.
+    For Actor, it's base model is ``actor.model`` and it's usually a ``transformers.PreTrainedModel``.
+    For Critic and RewardModel, it's base model is itself.
+
+    Args:
+        model (nn.Module): model to get base model from
+
+    Returns:
+        nn.Module: the base model
+    """
+    if isinstance(model, Actor):
+        return model.get_base_model()
+    return model
+
+
+__all__ = ['Actor', 'Critic', 'RewardModel', 'get_base_model']