SDK for session (#2312)

### What problem does this PR solve?

SDK for session
#1102 

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

---------

Co-authored-by: Feiue <10215101452@stu.ecun.edu.cn>
Co-authored-by: Kevin Hu <kevinhu.sh@gmail.com>

sdk/python/ragflow/ragflow.py

@@ -17,7 +17,6 @@ from typing import List
 
 import requests
 
-
 from .modules.chat_assistant import Assistant
 from .modules.dataset import DataSet
 
@@ -88,7 +87,7 @@ class RAGFlow:
             datasets.append(dataset.to_json())
 
         if llm is None:
-            llm = Assistant.LLM(self, {"model_name": "deepseek-chat",
+            llm = Assistant.LLM(self, {"model_name": None,
                                        "temperature": 0.1,
                                        "top_p": 0.3,
                                        "presence_penalty": 0.4,
@@ -142,4 +141,4 @@ class RAGFlow:
             for data in res['data']:
                 result_list.append(Assistant(self, data))
             return result_list
-        raise Exception(res["retmsg"])
+        raise Exception(res["retmsg"])