Fix:Set CUDA_VISIBLE_DEVICES In DefaultEmbedding (#7465)

### What problem does this PR solve?
https://github.com/infiniflow/ragflow/issues/7420

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

rag/llm/embedding_model.py

@@ -57,6 +57,7 @@ class Base(ABC):
 
 
 class DefaultEmbedding(Base):
+    os.environ['CUDA_VISIBLE_DEVICES'] = '0'
     _model = None
     _model_name = ""
     _model_lock = threading.Lock()