Update rerank

Signed-off-by: shiyu22 <shiyu.chen@zilliz.com>

rerank.py

@@ -1,17 +1,18 @@
 from typing import List
-from xml.dom.expatbuilder import theDOMImplementation
 
 from sentence_transformers import CrossEncoder
 from towhee.operator import NNOperator
 
 
 class ReRank(NNOperator):
-    def __init__(self, model_name: str = 'cross-encoder/ms-marco-MiniLM-L-12-v2'):
+    def __init__(self, model_name: str = 'cross-encoder/ms-marco-MiniLM-L-6-v2'):
         super().__init__()
         self._model_name = model_name
         self._model = CrossEncoder(self._model_name, max_length=1000)
 
     def __call__(self, query: str, docs: List, threshold: float = None):
+        if len(docs) == 0:
+            return [], []
         scores = self._model.predict([(query, doc) for doc in docs])
         re_ids = sorted(range(len(scores)), key=lambda k: scores[k], reverse=True)
         if threshold is None: