Add PyOp as base

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

milvus.py

@@ -1,11 +1,12 @@
 import numpy as np
 from towhee import register
 from urllib.parse import urlsplit
+from towhee.operator import PyOperator
 from pymilvus import connections, Collection
 
 
 @register(output_schema=['mr'])
-class Milvus:
+class Milvus(PyOperator):
     """
     Milvus ANN index class.
     """