[MINOR][ML] Minor correction in the powerIterationSuite

## What changes were proposed in this pull request?

Currently the power iteration clustering test in  spark ml, maps the results to the labels 0 and 1 for assertion. Since the clustering outputs need not be the same as the mapped labels, it may cause failure in the test case. Even if it correctly maps, theoretically we cannot guarantee which set belongs to which cluster label. KMeans can assign label 0 to either of the set.

PowerIterationClusteringSuite in the MLLib checks the clustering results without mapping to the particular cluster label, as shown below.
``  val predictions = Array.fill(2)(mutable.Set.empty[Long])
    model.assignments.collect().foreach { a =>
      predictions(a.cluster) += a.id
    }
    assert(predictions.toSet == Set((0 until n1).toSet, (n1 until n).toSet))
``

## How was this patch tested?
Existing tests

Author: Shahid <[email protected]>

Closes #21689 from shahidki31/picTestSuiteMinorCorrection.

Author: shahidki31

mllib/src/test/scala/org/apache/spark/ml/clustering/PowerIterationClusteringSuite.scala

@@ -17,6 +17,8 @@
 
 package org.apache.spark.ml.clustering
 
+import scala.collection.mutable
+
 import org.apache.spark.{SparkException, SparkFunSuite}
 import org.apache.spark.ml.util.DefaultReadWriteTest
 import org.apache.spark.mllib.util.MLlibTestSparkContext
@@ -76,23 +78,31 @@ class PowerIterationClusteringSuite extends SparkFunSuite
       .setMaxIter(40)
       .setWeightCol("weight")
       .assignClusters(data)
-    val localAssignments = assignments
-      .select('id, 'cluster)
-      .as[(Long, Int)].collect().toSet
-    val expectedResult = (0 until n1).map(x => (x, 1)).toSet ++
-      (n1 until n).map(x => (x, 0)).toSet
-    assert(localAssignments === expectedResult)
+      .select("id", "cluster")
+      .as[(Long, Int)]
+      .collect()
+
+    val predictions = Array.fill(2)(mutable.Set.empty[Long])
+    assignments.foreach {
+      case (id, cluster) => predictions(cluster) += id
+    }
+    assert(predictions.toSet === Set((0 until n1).toSet, (n1 until n).toSet))
 
     val assignments2 = new PowerIterationClustering()
       .setK(2)
       .setMaxIter(10)
       .setInitMode("degree")
       .setWeightCol("weight")
       .assignClusters(data)
-    val localAssignments2 = assignments2
-      .select('id, 'cluster)
-      .as[(Long, Int)].collect().toSet
-    assert(localAssignments2 === expectedResult)
+      .select("id", "cluster")
+      .as[(Long, Int)]
+      .collect()
+
+    val predictions2 = Array.fill(2)(mutable.Set.empty[Long])
+    assignments2.foreach {
+      case (id, cluster) => predictions2(cluster) += id
+    }
+    assert(predictions2.toSet === Set((0 until n1).toSet, (n1 until n).toSet))
   }
 
   test("supported input types") {