fix swift preprocessor (#1082)

Author: Jintao-Huang

swift/llm/utils/preprocess.py

@@ -19,7 +19,7 @@ def __call__(self, dataset: HfDataset) -> HfDataset:
             history: List[History] = []
             for h in tqdm(old_history):
                 if isinstance(h, str):
-                    h = ast.literal_eval(old_h)
+                    h = ast.literal_eval(h)
                 elif h is None:
                     h = []
                 if len(h) > 0: