[SPARK-22832][ML] BisectingKMeans unpersist unused datasets

## What changes were proposed in this pull request?
unpersist unused datasets

## How was this patch tested?
existing tests and local check in Spark-Shell

Author: Zheng RuiFeng <[email protected]>

Closes #20017 from zhengruifeng/bkm_unpersist.

Author: zhengruifeng

mllib/src/main/scala/org/apache/spark/mllib/clustering/BisectingKMeans.scala

@@ -197,7 +197,9 @@ class BisectingKMeans private (
           newClusters = summarize(d, newAssignments)
           newClusterCenters = newClusters.mapValues(_.center).map(identity)
         }
-        if (preIndices != null) preIndices.unpersist()
+        if (preIndices != null) {
+          preIndices.unpersist(false)
+        }
         preIndices = indices
         indices = updateAssignments(assignments, divisibleIndices, newClusterCenters).keys
           .persist(StorageLevel.MEMORY_AND_DISK)
@@ -212,7 +214,13 @@ class BisectingKMeans private (
       }
       level += 1
     }
-    if(indices != null) indices.unpersist()
+    if (preIndices != null) {
+      preIndices.unpersist(false)
+    }
+    if (indices != null) {
+      indices.unpersist(false)
+    }
+    norms.unpersist(false)
     val clusters = activeClusters ++ inactiveClusters
     val root = buildTree(clusters)
     new BisectingKMeansModel(root)