[SPARK-19384][ML] forget unpersist input dataset in IsotonicRegression

## What changes were proposed in this pull request?
unpersist the input dataset if `handlePersistence` = true

## How was this patch tested?
existing tests

Author: Zheng RuiFeng <ruifengz@foxmail.com>

Closes #16718 from zhengruifeng/isoReg_unpersisit.

mllib/src/main/scala/org/apache/spark/ml/regression/IsotonicRegression.scala

@@ -175,6 +175,8 @@ class IsotonicRegression @Since("1.5.0") (@Since("1.5.0") override val uid: Stri
     val isotonicRegression = new MLlibIsotonicRegression().setIsotonic($(isotonic))
     val oldModel = isotonicRegression.run(instances)
 
+    if (handlePersistence) instances.unpersist()
+
     val model = copyValues(new IsotonicRegressionModel(uid, oldModel).setParent(this))
     instr.logSuccess(model)
     model