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>

rag/nlp/__init__.py

@@ -18,7 +18,7 @@ import logging
 import random
 from collections import Counter
 
-from rag.utils import num_tokens_from_string
+from common.token_utils import num_tokens_from_string
 from . import rag_tokenizer
 import re
 import copy