Move token related functions to common (#10942)

### What problem does this PR solve?

As title

### Type of change

- [x] Refactoring

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

api/db/services/llm_service.py

@@ -16,7 +16,7 @@
 import inspect
 import logging
 import re
-from rag.utils import num_tokens_from_string
+from common.token_utils import num_tokens_from_string
 from functools import partial
 from typing import Generator
 from api.db.db_models import LLM