fix mem leak for local reranker (#1295)

### What problem does this PR solve?

#1288
### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

rag/llm/rerank_model.py

@@ -39,6 +39,7 @@ class Base(ABC):
 class DefaultRerank(Base):
     _model = None
     _model_lock = threading.Lock()
+
     def __init__(self, key, model_name, **kwargs):
         """
         If you have trouble downloading HuggingFace models, -_^ this might help!!
@@ -102,19 +103,24 @@ class JinaRerank(Base):
 
 class YoudaoRerank(DefaultRerank):
     _model = None
+    _model_lock = threading.Lock()
 
     def __init__(self, key=None, model_name="maidalun1020/bce-reranker-base_v1", **kwargs):
         from BCEmbedding import RerankerModel
         if not YoudaoRerank._model:
-            try:
-                print("LOADING BCE...")
-                YoudaoRerank._model = RerankerModel(model_name_or_path=os.path.join(
-                    get_home_cache_dir(),
-                    re.sub(r"^[a-zA-Z]+/", "", model_name)))
-            except Exception as e:
-                YoudaoRerank._model = RerankerModel(
-                    model_name_or_path=model_name.replace(
-                        "maidalun1020", "InfiniFlow"))
+            with YoudaoRerank._model_lock:
+                if not YoudaoRerank._model:
+                    try:
+                        print("LOADING BCE...")
+                        YoudaoRerank._model = RerankerModel(model_name_or_path=os.path.join(
+                            get_home_cache_dir(),
+                            re.sub(r"^[a-zA-Z]+/", "", model_name)))
+                    except Exception as e:
+                        YoudaoRerank._model = RerankerModel(
+                            model_name_or_path=model_name.replace(
+                                "maidalun1020", "InfiniFlow"))
+
+        self._model = YoudaoRerank._model
     
     def similarity(self, query: str, texts: list):
         pairs = [(query, truncate(t, self._model.max_length)) for t in texts]