[template] fix _extend_tokens idx_list (#5563)

Author: Jintao-Huang

swift/llm/template/base.py

@@ -304,7 +304,11 @@ def _extend_tokens(
             get_new_tokens: Callable[[int], List[int]]) -> Tuple[List[int], Optional[List[int]], Optional[List[float]]]:
         added_tokens_len = 0
         for i, idx in enumerate(replace_idx_list):
-            new_tokens = get_new_tokens(i)
+            try:
+                new_tokens = get_new_tokens(i)
+            except IndexError as e:
+                logger.warning(f'IndexError occurs in the _extend_tokens function: {e}.')
+                continue
             token_len = len(new_tokens)
             input_ids = input_ids[:idx + added_tokens_len] + new_tokens + input_ids[added_tokens_len + idx + 1:]
             if labels: