Import rag_tokenizer from Infinity (#11647)

### What problem does this PR solve?

- Original rag/nlp/rag_tokenizer.py is put to Infinity and infinity-sdk
via https://github.com/infiniflow/infinity/pull/3117 .
Import rag_tokenizer from infinity and inherit from
rag_tokenizer.RagTokenizer in new rag/nlp/rag_tokenizer.py.

- Bump infinity to 0.6.8

### Type of change
- [x] Refactoring

pyproject.toml

@@ -49,7 +49,7 @@ dependencies = [
     "html-text==0.6.2",
     "httpx[socks]>=0.28.1,<0.29.0",
     "huggingface-hub>=0.25.0,<0.26.0",
-    "infinity-sdk==0.6.7",
+    "infinity-sdk==0.6.8",
     "infinity-emb>=0.0.66,<0.0.67",
     "itsdangerous==2.1.2",
     "json-repair==0.35.0",
@@ -170,9 +170,6 @@ test = [
     "requests-toolbelt>=1.0.0",
 ]
 
-[[tool.uv.index]]
-url = "https://mirrors.aliyun.com/pypi/simple"
-
 [[tool.uv.index]]
 url = "https://pypi.tuna.tsinghua.edu.cn/simple"