Refactor Chat API (#2804)

### What problem does this PR solve?

Refactor Chat API

### Type of change

- [x] Refactoring

---------

Co-authored-by: liuhua <10215101452@stu.ecun.edu.cn>

sdk/python/ragflow/modules/chat.py

@@ -0,0 +1,78 @@
+from typing import List
+
+from .base import Base
+from .session import Session
+
+
+class Chat(Base):
+    def __init__(self, rag, res_dict):
+        self.id = ""
+        self.name = "assistant"
+        self.avatar = "path/to/avatar"
+        self.knowledgebases = ["kb1"]
+        self.llm = Chat.LLM(rag, {})
+        self.prompt = Chat.Prompt(rag, {})
+        super().__init__(rag, res_dict)
+
+    class LLM(Base):
+        def __init__(self, rag, res_dict):
+            self.model_name = "deepseek-chat"
+            self.temperature = 0.1
+            self.top_p = 0.3
+            self.presence_penalty = 0.4
+            self.frequency_penalty = 0.7
+            self.max_tokens = 512
+            super().__init__(rag, res_dict)
+
+    class Prompt(Base):
+        def __init__(self, rag, res_dict):
+            self.similarity_threshold = 0.2
+            self.keywords_similarity_weight = 0.7
+            self.top_n = 8
+            self.variables = [{"key": "knowledge", "optional": True}]
+            self.rerank_model = None
+            self.empty_response = None
+            self.opener = "Hi! I'm your assistant, what can I do for you?"
+            self.show_quote = True
+            self.prompt = (
+                "You are an intelligent assistant. Please summarize the content of the knowledge base to answer the question. "
+                "Please list the data in the knowledge base and answer in detail. When all knowledge base content is irrelevant to the question, "
+                "your answer must include the sentence 'The answer you are looking for is not found in the knowledge base!' "
+                "Answers need to consider chat history.\nHere is the knowledge base:\n{knowledge}\nThe above is the knowledge base."
+            )
+            super().__init__(rag, res_dict)
+
+    def update(self, update_message: dict):
+        res = self.put(f'/chat/{self.id}',
+                        update_message)
+        res = res.json()
+        if res.get("code") != 0:
+            raise Exception(res["message"])
+
+
+    def create_session(self, name: str = "New session") -> Session:
+        res = self.post("/session/save", {"name": name, "assistant_id": self.id})
+        res = res.json()
+        if res.get("retmsg") == "success":
+            return Session(self.rag, res['data'])
+        raise Exception(res["retmsg"])
+
+    def list_session(self) -> List[Session]:
+        res = self.get('/session/list', {"assistant_id": self.id})
+        res = res.json()
+        if res.get("retmsg") == "success":
+            result_list = []
+            for data in res["data"]:
+                result_list.append(Session(self.rag, data))
+            return result_list
+        raise Exception(res["retmsg"])
+
+    def get_session(self, id) -> Session:
+        res = self.get("/session/get", {"id": id,"assistant_id":self.id})
+        res = res.json()
+        if res.get("retmsg") == "success":
+            return Session(self.rag, res["data"])
+        raise Exception(res["retmsg"])
+
+    def get_prologue(self):
+        return self.prompt.opener