[SPARK-16079][PYSPARK][ML] Added missing import for DecisionTreeRegressionModel used in GBTClassificationModel

## What changes were proposed in this pull request?

Fixed missing import for DecisionTreeRegressionModel used in GBTClassificationModel trees method.

## How was this patch tested?

Local tests

Author: Bryan Cutler <[email protected]>

Closes #13787 from BryanCutler/pyspark-GBTClassificationModel-import-SPARK-16079.

Author: BryanCutler

python/pyspark/ml/classification.py

@@ -21,8 +21,8 @@
 from pyspark import since, keyword_only
 from pyspark.ml import Estimator, Model
 from pyspark.ml.param.shared import *
-from pyspark.ml.regression import (
-    RandomForestParams, TreeEnsembleParams, DecisionTreeModel, TreeEnsembleModels)
+from pyspark.ml.regression import DecisionTreeModel, DecisionTreeRegressionModel, \
+    RandomForestParams, TreeEnsembleModels, TreeEnsembleParams
 from pyspark.ml.util import *
 from pyspark.ml.wrapper import JavaEstimator, JavaModel, JavaParams
 from pyspark.ml.wrapper import JavaWrapper
@@ -798,6 +798,8 @@ class GBTClassifier(JavaEstimator, HasFeaturesCol, HasLabelCol, HasPredictionCol
     True
     >>> model.treeWeights == model2.treeWeights
     True
+    >>> model.trees
+    [DecisionTreeRegressionModel (uid=...) of depth..., DecisionTreeRegressionModel...]
 
     .. versionadded:: 1.4.0
     """

python/pyspark/ml/regression.py

@@ -994,6 +994,8 @@ class GBTRegressor(JavaEstimator, HasFeaturesCol, HasLabelCol, HasPredictionCol,
     True
     >>> model.treeWeights == model2.treeWeights
     True
+    >>> model.trees
+    [DecisionTreeRegressionModel (uid=...) of depth..., DecisionTreeRegressionModel...]
 
     .. versionadded:: 1.4.0
     """