Make fast embed and default embed mutually exclusive. (#4121)

### What problem does this PR solve?


### Type of change

- [x] Performance Improvement

rag/llm/embedding_model.py

         return np.array(res["embedding"]), 128
 
 
-class FastEmbed(Base):
-    _model = None
-    _model_name = ""
-    _model_lock = threading.Lock()
-
+class FastEmbed(DefaultEmbedding):
+    
     def __init__(
             self,
             key: str | None = None,
         if not settings.LIGHTEN and not FastEmbed._model:
             with FastEmbed._model_lock:
                 from fastembed import TextEmbedding
-                if not FastEmbed._model or model_name != FastEmbed._model_name:
+                if not DefaultEmbedding._model or model_name != DefaultEmbedding._model_name:
                     try:
-                        FastEmbed._model = TextEmbedding(model_name, cache_dir, threads, **kwargs)
-                        FastEmbed._model_name = model_name
+                        DefaultEmbedding._model = TextEmbedding(model_name, cache_dir, threads, **kwargs)
+                        DefaultEmbedding._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
+                        DefaultEmbedding._model = TextEmbedding(model_name, cache_dir, threads, **kwargs)
+        self._model = DefaultEmbedding._model
         self._model_name = model_name
 
     def encode(self, texts: list):