add support for mistral (#1153)

### What problem does this PR solve?

#433 

### Type of change

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

rag/llm/chat_model.py

@@ -472,3 +472,57 @@ class MiniMaxChat(Base):
         if not base_url:
             base_url="https://api.minimax.chat/v1/text/chatcompletion_v2"
         super().__init__(key, model_name, base_url)
+
+
+class MistralChat(Base):
+
+    def __init__(self, key, model_name, base_url=None):
+        from mistralai.client import MistralClient
+        self.client = MistralClient(api_key=key)
+        self.model_name = model_name
+
+    def chat(self, system, history, gen_conf):
+        if system:
+            history.insert(0, {"role": "system", "content": system})
+        for k in list(gen_conf.keys()):
+            if k not in ["temperature", "top_p", "max_tokens"]:
+                del gen_conf[k]
+        try:
+            response = self.client.chat(
+                model=self.model_name,
+                messages=history,
+                **gen_conf)
+            ans = response.choices[0].message.content
+            if response.choices[0].finish_reason == "length":
+                ans += "...\nFor the content length reason, it stopped, continue?" if is_english(
+                    [ans]) else "······\n由于长度的原因，回答被截断了，要继续吗？"
+            return ans, response.usage.total_tokens
+        except openai.APIError as e:
+            return "**ERROR**: " + str(e), 0
+
+    def chat_streamly(self, system, history, gen_conf):
+        if system:
+            history.insert(0, {"role": "system", "content": system})
+        for k in list(gen_conf.keys()):
+            if k not in ["temperature", "top_p", "max_tokens"]:
+                del gen_conf[k]
+        ans = ""
+        total_tokens = 0
+        try:
+            response = self.client.chat_stream(
+                model=self.model_name,
+                messages=history,
+                **gen_conf)
+            for resp in response:
+                if not resp.choices or not resp.choices[0].delta.content:continue
+                ans += resp.choices[0].delta.content
+                total_tokens += 1
+                if resp.choices[0].finish_reason == "length":
+                    ans += "...\nFor the content length reason, it stopped, continue?" if is_english(
+                        [ans]) else "······\n由于长度的原因，回答被截断了，要继续吗？"
+                yield ans
+
+        except openai.APIError as e:
+            yield ans + "\n**ERROR**: " + str(e)
+
+        yield total_tokens