Fix fastembed reloading issue. (#4117)

### What problem does this PR solve?


### Type of change

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

rag/llm/embedding_model.py

@@ -47,6 +47,7 @@ def encode_queries(self, text: str):
 
 class DefaultEmbedding(Base):
     _model = None
+    _model_name = ""
     _model_lock = threading.Lock()
     def __init__(self, key, model_name, **kwargs):
         """
@@ -69,6 +70,7 @@ def __init__(self, key, model_name, **kwargs):
                         DefaultEmbedding._model = FlagModel(os.path.join(get_home_cache_dir(), re.sub(r"^[a-zA-Z0-9]+/", "", model_name)),
                                                             query_instruction_for_retrieval="为这个句子生成表示以用于检索相关文章：",
                                                             use_fp16=torch.cuda.is_available())
+                        DefaultEmbedding._model_name = model_name
                     except Exception:
                         model_dir = snapshot_download(repo_id="BAAI/bge-large-zh-v1.5",
                                                       local_dir=os.path.join(get_home_cache_dir(), re.sub(r"^[a-zA-Z0-9]+/", "", model_name)),
@@ -77,6 +79,7 @@ def __init__(self, key, model_name, **kwargs):
                                                             query_instruction_for_retrieval="为这个句子生成表示以用于检索相关文章：",
                                                             use_fp16=torch.cuda.is_available())
         self._model = DefaultEmbedding._model
+        self._model_name = DefaultEmbedding._model_name
 
     def encode(self, texts: list):
         batch_size = 16
@@ -250,6 +253,8 @@ def encode_queries(self, text):
 
 class FastEmbed(Base):
     _model = None
+    _model_name = ""
+    _model_lock = threading.Lock()
 
     def __init__(
             self,
@@ -260,8 +265,20 @@ def __init__(
             **kwargs,
     ):
         if not settings.LIGHTEN and not FastEmbed._model:
-            from fastembed import TextEmbedding
-            self._model = TextEmbedding(model_name, cache_dir, threads, **kwargs)
+            with FastEmbed._model_lock:
+                from fastembed import TextEmbedding
+                if not FastEmbed._model or model_name != FastEmbed._model_name:
+                    try:
+                        FastEmbed._model = TextEmbedding(model_name, cache_dir, threads, **kwargs)
+                        FastEmbed._model_name = model_name
+                    except Exception:
+                        cache_dir = snapshot_download(repo_id="BAAI/bge-small-en-v1.5",
+                                                      local_dir=os.path.join(get_home_cache_dir(),
+                                                                             re.sub(r"^[a-zA-Z0-9]+/", "", model_name)),
+                                                      local_dir_use_symlinks=False)
+                        FastEmbed._model = TextEmbedding(model_name, cache_dir, threads, **kwargs)
+        self._model = FastEmbed._model
+        self._model_name = model_name
 
     def encode(self, texts: list):
         # Using the internal tokenizer to encode the texts and get the total