[ML-3869] Make Quantilediscretizer work with spark-2.3 (#154)

Add setOutputCol for Quantilediscretizer so that it works for spark-2.3.

The code has been manually tested by change the spark version.

src/main/scala/com/databricks/spark/sql/perf/mllib/feature/QuantileDiscretizer.scala

@@ -31,6 +31,7 @@ object QuantileDiscretizer extends BenchmarkAlgorithm with TestFromTraining with
     import ctx.params._
     new ml.feature.QuantileDiscretizer()
       .setInputCol(inputCol)
+      .setOutputCol(outputCol)
       .setNumBuckets(bucketizerNumBuckets)
       .setRelativeError(relativeError)
   }

src/main/scala/com/databricks/spark/sql/perf/mllib/feature/UnaryTransformer.scala

@@ -3,4 +3,5 @@ package com.databricks.spark.sql.perf.mllib.feature
 /** Trait defining common state/methods for featurizers taking a single input col */
 private[feature] trait UnaryTransformer {
   private[feature] val inputCol = "inputCol"
+  private[feature] val outputCol = "outputCol"
 }