refactor code (#583)

### What problem does this PR solve?

### Type of change

- [x] Refactoring

rag/llm/embedding_model.py

@@ -56,7 +56,7 @@ class Base(ABC):
         raise NotImplementedError("Please implement encode method!")
 
 
-class HuEmbedding(Base):
+class DefaultEmbedding(Base):
     def __init__(self, *args, **kwargs):
         """
         If you have trouble downloading HuggingFace models, -_^ this might help!!