[MINOR] Correct validateAndTransformSchema in GaussianMixture and AFTSurvivalRegression

## What changes were proposed in this pull request?

The line SchemaUtils.appendColumn(schema, $(predictionCol), IntegerType) did not modify the variable schema, hence only the last line had any effect. A temporary variable is used to correctly append the two columns predictionCol and probabilityCol.

## How was this patch tested?

Manually.

Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: Cédric Pelvet <[email protected]>

Closes #18980 from sharp-pixel/master.

Author: sharp-pixel

mllib/src/main/scala/org/apache/spark/ml/clustering/GaussianMixture.scala

@@ -64,8 +64,8 @@ private[clustering] trait GaussianMixtureParams extends Params with HasMaxIter w
    */
   protected def validateAndTransformSchema(schema: StructType): StructType = {
     SchemaUtils.checkColumnType(schema, $(featuresCol), new VectorUDT)
-    SchemaUtils.appendColumn(schema, $(predictionCol), IntegerType)
-    SchemaUtils.appendColumn(schema, $(probabilityCol), new VectorUDT)
+    val schemaWithPredictionCol = SchemaUtils.appendColumn(schema, $(predictionCol), IntegerType)
+    SchemaUtils.appendColumn(schemaWithPredictionCol, $(probabilityCol), new VectorUDT)
   }
 }
 

mllib/src/main/scala/org/apache/spark/ml/regression/AFTSurvivalRegression.scala

@@ -109,10 +109,12 @@ private[regression] trait AFTSurvivalRegressionParams extends Params
       SchemaUtils.checkNumericType(schema, $(censorCol))
       SchemaUtils.checkNumericType(schema, $(labelCol))
     }
-    if (hasQuantilesCol) {
+
+    val schemaWithQuantilesCol = if (hasQuantilesCol) {
       SchemaUtils.appendColumn(schema, $(quantilesCol), new VectorUDT)
-    }
-    SchemaUtils.appendColumn(schema, $(predictionCol), DoubleType)
+    } else schema
+
+    SchemaUtils.appendColumn(schemaWithQuantilesCol, $(predictionCol), DoubleType)
   }
 }