[SPARK-22075][ML] GBTs unpersist datasets cached by Checkpointer

## What changes were proposed in this pull request?
`PeriodicRDDCheckpointer` will automatically persist the last 3 datasets called by `PeriodicRDDCheckpointer.update()`.
In GBTs, the last 3 intermediate rdds are still cached after `fit()`

## How was this patch tested?
existing tests and local test in spark-shell

Author: Zheng RuiFeng <[email protected]>

Closes apache#19288 from zhengruifeng/gbt_unpersist.

Author: zhengruifeng

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

@@ -360,7 +360,9 @@ private[spark] object GradientBoostedTrees extends Logging {
     logInfo("Internal timing for DecisionTree:")
     logInfo(s"$timer")
 
+    predErrorCheckpointer.unpersistDataSet()
     predErrorCheckpointer.deleteAllCheckpoints()
+    validatePredErrorCheckpointer.unpersistDataSet()
     validatePredErrorCheckpointer.deleteAllCheckpoints()
     if (persistedInput) input.unpersist()