fix iss#889:

when init_model on startup, set llm model's history_len to LLM_HISTORY_LEN from model_config.

webui.py

@@ -104,6 +104,7 @@ def init_model():
     args_dict = vars(args)
     shared.loaderCheckPoint = LoaderCheckPoint(args_dict)
     llm_model_ins = shared.loaderLLM()
+    llm_model_ins.history_len = LLM_HISTORY_LEN
     try:
         local_doc_qa.init_cfg(llm_model=llm_model_ins)
         answer_result_stream_result = local_doc_qa.llm_model_chain(