Fix DPO loss_type to use Enum instead of Literal for schema generation

- Replace Literal["sigmoid", "hinge", "ipo", "kto_pair"] with DPOLossType enum
- Schema generator only supports Literal types with single values

llama_stack/apis/post_training/post_training.py

@@ -104,10 +104,18 @@ class RLHFAlgorithm(Enum):
     dpo = "dpo"
 
 
+@json_schema_type
+class DPOLossType(Enum):
+    sigmoid = "sigmoid"
+    hinge = "hinge"
+    ipo = "ipo"
+    kto_pair = "kto_pair"
+
+
 @json_schema_type
 class DPOAlignmentConfig(BaseModel):
     beta: float
-    loss_type: Literal["sigmoid", "hinge", "ipo", "kto_pair"] = "sigmoid"
+    loss_type: DPOLossType = DPOLossType.sigmoid
 
 
 @json_schema_type