[SPARK-9962] [ML] Decision Tree training: prevNodeIdsForInstances.unpersist() at end of training

NodeIdCache: prevNodeIdsForInstances.unpersist() needs to be called at end of training.

Author: Holden Karau <holden@pigscanfly.ca>

Closes #8541 from holdenk/SPARK-9962-decission-tree-training-prevNodeIdsForiNstances-unpersist-at-end-of-training.

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

@@ -164,10 +164,10 @@ private[spark] class NodeIdCache(
         }
       }
     }
-  }
-  if (prevNodeIdsForInstances != null) {
-    // Unpersist the previous one if one exists.
-    prevNodeIdsForInstances.unpersist()
+    if (prevNodeIdsForInstances != null) {
+      // Unpersist the previous one if one exists.
+      prevNodeIdsForInstances.unpersist()
+    }
   }
 }
 

mllib/src/main/scala/org/apache/spark/mllib/tree/impl/NodeIdCache.scala

@@ -166,6 +166,10 @@ private[spark] class NodeIdCache(
         fs.delete(new Path(old.getCheckpointFile.get), true)
       }
     }
+    if (prevNodeIdsForInstances != null) {
+      // Unpersist the previous one if one exists.
+      prevNodeIdsForInstances.unpersist()
+    }
   }
 }