adding developer api annotation for overriden methods

Author: manishamde

mllib/src/main/scala/org/apache/spark/mllib/tree/impurity/Entropy.scala

@@ -52,6 +52,14 @@ object Entropy extends Impurity {
     impurity
   }
 
+  /**
+   * :: DeveloperApi ::
+   * variance calculation
+   * @param count number of instances
+   * @param sum sum of labels
+   * @param sumSquares summation of squares of the labels
+   */
+  @DeveloperApi
   override def calculate(count: Double, sum: Double, sumSquares: Double): Double =
     throw new UnsupportedOperationException("Entropy.calculate")
 }

mllib/src/main/scala/org/apache/spark/mllib/tree/impurity/Gini.scala

@@ -48,6 +48,14 @@ object Gini extends Impurity {
     impurity
   }
 
+  /**
+   * :: DeveloperApi ::
+   * variance calculation
+   * @param count number of instances
+   * @param sum sum of labels
+   * @param sumSquares summation of squares of the labels
+   */
+  @DeveloperApi
   override def calculate(count: Double, sum: Double, sumSquares: Double): Double =
     throw new UnsupportedOperationException("Gini.calculate")
 }

mllib/src/main/scala/org/apache/spark/mllib/tree/impurity/Variance.scala

@@ -25,7 +25,16 @@ import org.apache.spark.annotation.{DeveloperApi, Experimental}
  */
 @Experimental
 object Variance extends Impurity {
-   override def calculate(counts: Array[Double], totalCounts: Double): Double =
+
+  /**
+   * :: DeveloperApi ::
+   * information calculation for multiclass classification
+   * @param counts Array[Double] with counts for each label
+   * @param totalCount sum of counts for all labels
+   * @return information value
+   */
+  @DeveloperApi
+  override def calculate(counts: Array[Double], totalCount: Double): Double =
      throw new UnsupportedOperationException("Variance.calculate")
 
   /**