refactor (#1124)

### What problem does this PR solve?


### Type of change

- [x] Refactoring

api/db/services/dialog_service.py

@@ -112,14 +112,15 @@ def chat(dialog, messages, stream=True, **kwargs):
             prompt_config["system"] = prompt_config["system"].replace(
                 "{%s}" % p["key"], " ")
 
+    rerank_mdl = None
+    if dialog.rerank_id:
+        rerank_mdl = LLMBundle(dialog.tenant_id, LLMType.RERANK, dialog.rerank_id)
+
     for _ in range(len(questions) // 2):
         questions.append(questions[-1])
     if "knowledge" not in [p["key"] for p in prompt_config["parameters"]]:
         kbinfos = {"total": 0, "chunks": [], "doc_aggs": []}
     else:
-        rerank_mdl = None
-        if dialog.rerank_id:
-            rerank_mdl = LLMBundle(dialog.tenant_id, LLMType.RERANK, dialog.rerank_id)
         kbinfos = retrievaler.retrieval(" ".join(questions), embd_mdl, dialog.tenant_id, dialog.kb_ids, 1, dialog.top_n,
                                         dialog.similarity_threshold,
                                         dialog.vector_similarity_weight,