Add rerank op

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

__init__.py

@@ -0,0 +1,4 @@
+from .rerank import ReRank
+
+def rerank(*args, **kwargs):
+    return ReRank(*args, **kwargs)

requirements.txt

@@ -0,0 +1 @@
+sentence_transformers

rerank.py

@@ -0,0 +1,18 @@
+import numpy as np
+from typing import List
+
+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'):
+        super().__init__()
+        self._model_name = model_name
+        self._model = CrossEncoder(self._model_name, max_length=1000)
+
+    def __call__(self, query: str, docs: List):
+        scores = self._model.predict([(query, doc) for doc in docs])
+        re_ids = sorted(range(len(scores)), key=lambda k: scores[k], reverse=True)
+        print(re_ids, docs)
+        re_docs = [docs[i] for i in re_ids]
+        return re_docs