Fix errors detected by Ruff (#3918)

### What problem does this PR solve?

Fix errors detected by Ruff

### Type of change

- [x] Refactoring

rag/raptor.py

@@ -47,7 +47,8 @@ class RecursiveAbstractiveProcessing4TreeOrganizedRetrieval:
     def __call__(self, chunks, random_state, callback=None):
         layers = [(0, len(chunks))]
         start, end = 0, len(chunks)
-        if len(chunks) <= 1: return
+        if len(chunks) <= 1:
+            return
         chunks = [(s, a) for s, a in chunks if len(a) > 0]
 
         def summarize(ck_idx, lock):
@@ -66,7 +67,8 @@ class RecursiveAbstractiveProcessing4TreeOrganizedRetrieval:
                 logging.debug(f"SUM: {cnt}")
                 embds, _ = self._embd_model.encode([cnt])
                 with lock:
-                    if not len(embds[0]): return
+                    if not len(embds[0]):
+                        return
                     chunks.append((cnt, embds[0]))
             except Exception as e:
                 logging.exception("summarize got exception")