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/oss_conn.py

@@ -19,7 +19,7 @@ from botocore.exceptions import ClientError
 from botocore.config import Config
 import time
 from io import BytesIO
-from rag.utils import singleton
+from common.decorator import singleton
 from rag import settings