refactor model_dtype, fix PPO trainer

src/llmtuner/hparams/model_args.py

@@ -67,9 +67,9 @@ class ModelArguments:
         default=None,
         metadata={"help": "Auth token to log in with Hugging Face Hub."}
     )
-    layernorm_dtype: Optional[Literal["auto", "fp16", "bf16", "fp32"]] = field(
-        default="auto",
-        metadata={"help": "Data type of the layer norm weights."}
+    upcast_layernorm: Optional[bool] = field(
+        default=False,
+        metadata={"help": "Whether to upcast the layernorm weights in fp32."}
     )
 
     def __post_init__(self):