[SPARK-7047] [ML] ml.Model optional parent support

Made Model.parent transient.  Added Model.hasParent to test for null parent

CC: mengxr

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

Closes #5914 from jkbradley/parent-optional and squashes the following commits:

d501774 [Joseph K. Bradley] Made Model.parent transient.  Added Model.hasParent to test for null parent

mllib/src/main/scala/org/apache/spark/ml/Model.scala

@@ -32,7 +32,7 @@ abstract class Model[M <: Model[M]] extends Transformer {
    * The parent estimator that produced this model.
    * Note: For ensembles' component Models, this value can be null.
    */
-  var parent: Estimator[M] = _
+  @transient var parent: Estimator[M] = _
 
   /**
    * Sets the parent of this model (Java API).
@@ -42,6 +42,9 @@ abstract class Model[M <: Model[M]] extends Transformer {
     this.asInstanceOf[M]
   }
 
+  /** Indicates whether this [[Model]] has a corresponding parent. */
+  def hasParent: Boolean = parent != null
+
   override def copy(extra: ParamMap): M = {
     // The default implementation of Params.copy doesn't work for models.
     throw new NotImplementedError(s"${this.getClass} doesn't implement copy(extra: ParamMap)")

mllib/src/test/scala/org/apache/spark/ml/classification/LogisticRegressionSuite.scala

@@ -83,6 +83,7 @@ class LogisticRegressionSuite extends FunSuite with MLlibTestSparkContext {
     assert(model.getRawPredictionCol === "rawPrediction")
     assert(model.getProbabilityCol === "probability")
     assert(model.intercept !== 0.0)
+    assert(model.hasParent)
   }
 
   test("logistic regression doesn't fit intercept when fitIntercept is off") {

mllib/src/test/scala/org/apache/spark/ml/classification/RandomForestClassifierSuite.scala

@@ -162,5 +162,7 @@ private object RandomForestClassifierSuite {
     val oldModelAsNew = RandomForestClassificationModel.fromOld(
       oldModel, newModel.parent.asInstanceOf[RandomForestClassifier], categoricalFeatures)
     TreeTests.checkEqual(oldModelAsNew, newModel)
+    assert(newModel.hasParent)
+    assert(!newModel.trees.head.asInstanceOf[DecisionTreeClassificationModel].hasParent)
   }
 }