add experimental tag

Author: WeichenXu123

mllib/src/main/scala/org/apache/spark/ml/stat/Summarizer.scala

@@ -19,7 +19,7 @@ package org.apache.spark.ml.stat
 
 import java.io._
 
-import org.apache.spark.annotation.Since
+import org.apache.spark.annotation.{Experimental, Since}
 import org.apache.spark.internal.Logging
 import org.apache.spark.ml.linalg.{Vector, Vectors, VectorUDT}
 import org.apache.spark.sql.Column
@@ -35,8 +35,9 @@ import org.apache.spark.sql.types._
  * Users should not directly create such builders, but instead use one of the methods in
  * [[Summarizer]].
  */
+@Experimental
 @Since("2.3.0")
-abstract class SummaryBuilder {
+sealed abstract class SummaryBuilder {
   /**
    * Returns an aggregate object that contains the summary of the column with the requested metrics.
    * @param featuresCol a column that contains features Vector object.
@@ -73,6 +74,7 @@ abstract class SummaryBuilder {
  * Note: Currently, the performance of this interface is about 2x~3x slower then using the RDD
  * interface.
  */
+@Experimental
 @Since("2.3.0")
 object Summarizer extends Logging {
 
@@ -155,8 +157,7 @@ private[ml] class SummaryBuilderImpl(
   }
 }
 
-private[ml]
-object SummaryBuilderImpl extends Logging {
+private[ml] object SummaryBuilderImpl extends Logging {
 
   def implementedMetrics: Seq[String] = allMetrics.map(_._1).sorted