Move singleton to common directory (#10935)

### What problem does this PR solve?

As title

### Type of change

- [x] Refactoring

Signed-off-by: Jin Hai <haijin.chn@gmail.com>

rag/utils/minio_conn.py

@@ -21,7 +21,7 @@ from minio.commonconfig import CopySource
 from minio.error import S3Error
 from io import BytesIO
 from rag import settings
-from rag.utils import singleton
+from common.decorator import singleton
 
 
 @singleton