fix QWenSeq2txt bug (#2245)

### What problem does this PR solve?

#2243

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

api/apps/llm_app.py

     chat_passed, embd_passed, rerank_passed = False, False, False
     factory = req["llm_factory"]
     msg = ""
-    for llm in LLMService.query(fid=factory):
+    for llm in LLMService.query(fid=factory)[:3]:
         if not embd_passed and llm.model_type == LLMType.EMBEDDING.value:
             mdl = EmbeddingModel[factory](
                 req["api_key"], llm.llm_name, base_url=req.get("base_url"))

rag/llm/sequence2txt_model.py

         ans = ""
         if result.status_code == HTTPStatus.OK:
             for sentence in result.get_sentence():
-                ans += str(sentence + '\n')
+                ans += sentence.text.decode('utf-8') + '\n'
             return ans, num_tokens_from_string(ans)
 
         return "**ERROR**: " + result.message, 0