[SPARK-14386][ML] Changed spark.ml ensemble trees methods to return concrete types

## What changes were proposed in this pull request?

In spark.ml, GBT and RandomForest expose the trait DecisionTreeModel in the trees method, but they should not since it is a private trait (and not ready to be made public). It will also be more useful to users if we return the concrete types.

This PR: return concrete types

The MIMA checks appear to be OK with this change.

## How was this patch tested?

Existing unit tests

Author: Joseph K. Bradley <joseph@databricks.com>

Closes #12158 from jkbradley/hide-dtm.

1 files changed, 1 insertions, 1 deletions

diff --git a/mllib/src/test/scala/org/apache/spark/ml/tree/impl/TreeTests.scala b/mllib/src/test/scala/org/apache/spark/ml/tree/impl/TreeTests.scala
index bd5bd17147..b650a9f092 100644
--- a/mllib/src/test/scala/org/apache/spark/ml/tree/impl/TreeTests.scala
+++ b/mllib/src/test/scala/org/apache/spark/ml/tree/impl/TreeTests.scala
@@ -131,7 +131,7 @@ private[ml] object TreeTests extends SparkFunSuite {
    * Check if the two models are exactly the same.
    * If the models are not equal, this throws an exception.
    */
-  def checkEqual(a: TreeEnsembleModel, b: TreeEnsembleModel): Unit = {
+  def checkEqual[M <: DecisionTreeModel](a: TreeEnsembleModel[M], b: TreeEnsembleModel[M]): Unit = {
     try {
       a.trees.zip(b.trees).foreach { case (treeA, treeB) =>
         TreeTests.checkEqual(treeA, treeB)