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/redis_conn.py

@@ -20,7 +20,7 @@ import uuid
 
 import valkey as redis
 from rag import settings
-from rag.utils import singleton
+from common.decorator import singleton
 from valkey.lock import Lock
 import trio