[SPARK-7100] [MLLIB] Fix persisted RDD leak in GradientBoostTrees

This fixes a leak of a persisted RDD where GradientBoostTrees can call persist but never unpersists.

Jira: https://issues.apache.org/jira/browse/SPARK-7100

Discussion: http://apache-spark-developers-list.1001551.n3.nabble.com/GradientBoostTrees-leaks-a-persisted-RDD-td11750.html

Author: Jim Carroll <[email protected]>

Closes #5669 from jimfcarroll/gb-unpersist-fix and squashes the following commits:

45f4b03 [Jim Carroll] [SPARK-7100][MLLib] Fix persisted RDD leak in GradientBoostTrees

Author: Jim Carroll

mllib/src/main/scala/org/apache/spark/mllib/tree/GradientBoostedTrees.scala

@@ -177,9 +177,10 @@ object GradientBoostedTrees extends Logging {
     treeStrategy.assertValid()
 
     // Cache input
-    if (input.getStorageLevel == StorageLevel.NONE) {
+    val persistedInput = if (input.getStorageLevel == StorageLevel.NONE) {
       input.persist(StorageLevel.MEMORY_AND_DISK)
-    }
+      true
+    } else false
 
     timer.stop("init")
 
@@ -265,6 +266,9 @@ object GradientBoostedTrees extends Logging {
 
     logInfo("Internal timing for DecisionTree:")
     logInfo(s"$timer")
+
+    if (persistedInput) input.unpersist()
+    
     if (validate) {
       new GradientBoostedTreesModel(
         boostingStrategy.treeStrategy.algo,