[SPARK-25997][ML] add Python example code for Power Iteration Clustering in spark.ml

## What changes were proposed in this pull request?

Add python example for Power Iteration Clustering in spark.ml

## How was this patch tested?

Manually tested

Closes apache#22996 from huaxingao/spark-25997.

Authored-by: Huaxin Gao <[email protected]>
Signed-off-by: Sean Owen <[email protected]>

Author: huaxingao

docs/ml-clustering.md

@@ -298,6 +298,12 @@ Refer to the [Java API docs](api/java/org/apache/spark/ml/clustering/PowerIterat
 {% include_example java/org/apache/spark/examples/ml/JavaPowerIterationClusteringExample.java %}
 </div>
 
+<div data-lang="python" markdown="1">
+Refer to the [Python API docs](api/python/pyspark.ml.html#pyspark.ml.clustering.PowerIterationClustering) for more details.
+
+{% include_example python/ml/power_iteration_clustering_example.py %}
+</div>
+
 <div data-lang="r" markdown="1">
 
 Refer to the [R API docs](api/R/spark.powerIterationClustering.html) for more details.

examples/src/main/python/ml/power_iteration_clustering_example.py

@@ -0,0 +1,49 @@
+#
+# Licensed to the Apache Software Foundation (ASF) under one or more
+# contributor license agreements.  See the NOTICE file distributed with
+# this work for additional information regarding copyright ownership.
+# The ASF licenses this file to You under the Apache License, Version 2.0
+# (the "License"); you may not use this file except in compliance with
+# the License.  You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+"""
+An example demonstrating PowerIterationClustering.
+Run with:
+  bin/spark-submit examples/src/main/python/ml/power_iteration_clustering_example.py
+"""
+# $example on$
+from pyspark.ml.clustering import PowerIterationClustering
+# $example off$
+from pyspark.sql import SparkSession
+
+if __name__ == "__main__":
+    spark = SparkSession\
+        .builder\
+        .appName("PowerIterationClusteringExample")\
+        .getOrCreate()
+
+    # $example on$
+    df = spark.createDataFrame([
+        (0, 1, 1.0),
+        (0, 2, 1.0),
+        (1, 2, 1.0),
+        (3, 4, 1.0),
+        (4, 0, 0.1)
+    ], ["src", "dst", "weight"])
+
+    pic = PowerIterationClustering(k=2, maxIter=20, initMode="degree", weightCol="weight")
+
+    # Shows the cluster assignment
+    pic.assignClusters(df).show()
+    # $example off$
+
+    spark.stop()