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/flow/tokenizer/tokenizer.py

@@ -29,7 +29,7 @@ from rag.flow.tokenizer.schema import TokenizerFromUpstream
 from rag.nlp import rag_tokenizer
 from rag.settings import EMBEDDING_BATCH_SIZE
 from rag.svr.task_executor import embed_limiter
-from rag.utils import truncate
+from common.token_utils import truncate
 
 
 class TokenizerParam(ProcessParamBase):