refine reteival of multi-turn conversation (#2520)

### What problem does this PR solve?

#2362 #2484

### Type of change

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

api/db/services/dialog_service.py

@@ -149,6 +149,11 @@ def chat(dialog, messages, stream=True, **kwargs):
             prompt_config["system"] = prompt_config["system"].replace(
                 "{%s}" % p["key"], " ")
 
+    if len(questions) > 1 and prompt_config.get("refine_multiturn"):
+        questions = [full_question(dialog.tenant_id, dialog.llm_id, messages)]
+    else:
+        questions = questions[-1:]
+
     rerank_mdl = None
     if dialog.rerank_id:
         rerank_mdl = LLMBundle(dialog.tenant_id, LLMType.RERANK, dialog.rerank_id)
@@ -410,6 +415,58 @@ def rewrite(tenant_id, llm_id, question):
     return ans
 
 
+def full_question(tenant_id, llm_id, messages):
+    if llm_id2llm_type(llm_id) == "image2text":
+        chat_mdl = LLMBundle(tenant_id, LLMType.IMAGE2TEXT, llm_id)
+    else:
+        chat_mdl = LLMBundle(tenant_id, LLMType.CHAT, llm_id)
+    conv = []
+    for m in messages:
+        if m["role"] not in ["user", "assistant"]: continue
+        conv.append("{}: {}".format(m["role"].upper(), m["content"]))
+    conv = "\n".join(conv)
+    prompt = f"""
+Role: A helpful assistant
+Task: Generate a full user question that would follow the conversation.
+Requirements & Restrictions:
+  - Text generated MUST be in the same language of the original user's question.
+  - If the user's latest question is completely, don't do anything, just return the original question.
+  - DON'T generate anything except a refined question.
+
+######################
+-Examples-
+######################
+
+# Example 1
+## Conversation
+USER: What is the name of Donald Trump's father?
+ASSISTANT:  Fred Trump.
+USER: And his mother?
+###############
+Output: What's the name of Donald Trump's mother?
+
+------------
+# Example 2
+## Conversation
+USER: What is the name of Donald Trump's father?
+ASSISTANT:  Fred Trump.
+USER: And his mother?
+ASSISTANT:  Mary Trump.
+User: What's her full name?
+###############
+Output: What's the full name of Donald Trump's mother Mary Trump?
+
+######################
+
+# Real Data
+## Conversation
+{conv}
+###############
+    """
+    ans = chat_mdl.chat(prompt, [{"role": "user", "content": "Output: "}], {"temperature": 0.2})
+    return ans if ans.find("**ERROR**") < 0 else messages[-1]["content"]
+
+
 def tts(tts_mdl, text):
     if not tts_mdl or not text: return
     bin = b""