[SPARK-23377][ML] Fixes Bucketizer with multiple columns persistence bug

## What changes were proposed in this pull request?

#### Problem:

Since 2.3, `Bucketizer` supports multiple input/output columns. We will check if exclusive params are set during transformation. E.g., if `inputCols` and `outputCol` are both set, an error will be thrown.

However, when we write `Bucketizer`, looks like the default params and user-supplied params are merged during writing. All saved params are loaded back and set to created model instance. So the default `outputCol` param in `HasOutputCol` trait will be set in `paramMap` and become an user-supplied param. That makes the check of exclusive params failed.

#### Fix:

This changes the saving logic of Bucketizer to handle this case. This is a quick fix to catch the time of 2.3. We should consider modify the persistence mechanism later.

Please see the discussion in the JIRA.

Note: The multi-column `QuantileDiscretizer` also has the same issue.

## How was this patch tested?

Modified tests.

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #20594 from viirya/SPARK-23377-2.

mllib/src/main/scala/org/apache/spark/ml/feature/Bucketizer.scala

@@ -19,6 +19,10 @@ package org.apache.spark.ml.feature
 
 import java.{util => ju}
 
+import org.json4s.JsonDSL._
+import org.json4s.JValue
+import org.json4s.jackson.JsonMethods._
+
 import org.apache.spark.SparkException
 import org.apache.spark.annotation.Since
 import org.apache.spark.ml.Model
@@ -213,6 +217,8 @@ final class Bucketizer @Since("1.4.0") (@Since("1.4.0") override val uid: String
   override def copy(extra: ParamMap): Bucketizer = {
     defaultCopy[Bucketizer](extra).setParent(parent)
   }
+
+  override def write: MLWriter = new Bucketizer.BucketizerWriter(this)
 }
 
 @Since("1.6.0")
@@ -290,6 +296,28 @@ object Bucketizer extends DefaultParamsReadable[Bucketizer] {
     }
   }
 
+
+  private[Bucketizer] class BucketizerWriter(instance: Bucketizer) extends MLWriter {
+
+    override protected def saveImpl(path: String): Unit = {
+      // SPARK-23377: The default params will be saved and loaded as user-supplied params.
+      // Once `inputCols` is set, the default value of `outputCol` param causes the error
+      // when checking exclusive params. As a temporary to fix it, we skip the default value
+      // of `outputCol` if `inputCols` is set when saving the metadata.
+      // TODO: If we modify the persistence mechanism later to better handle default params,
+      // we can get rid of this.
+      var paramWithoutOutputCol: Option[JValue] = None
+      if (instance.isSet(instance.inputCols)) {
+        val params = instance.extractParamMap().toSeq
+        val jsonParams = params.filter(_.param != instance.outputCol).map { case ParamPair(p, v) =>
+          p.name -> parse(p.jsonEncode(v))
+        }.toList
+        paramWithoutOutputCol = Some(render(jsonParams))
+      }
+      DefaultParamsWriter.saveMetadata(instance, path, sc, paramMap = paramWithoutOutputCol)
+    }
+  }
+
   @Since("1.6.0")
   override def load(path: String): Bucketizer = super.load(path)
 }

mllib/src/main/scala/org/apache/spark/ml/feature/QuantileDiscretizer.scala

@@ -17,6 +17,10 @@
 
 package org.apache.spark.ml.feature
 
+import org.json4s.JsonDSL._
+import org.json4s.JValue
+import org.json4s.jackson.JsonMethods._
+
 import org.apache.spark.annotation.Since
 import org.apache.spark.internal.Logging
 import org.apache.spark.ml._
@@ -249,11 +253,35 @@ final class QuantileDiscretizer @Since("1.6.0") (@Since("1.6.0") override val ui
 
   @Since("1.6.0")
   override def copy(extra: ParamMap): QuantileDiscretizer = defaultCopy(extra)
+
+  override def write: MLWriter = new QuantileDiscretizer.QuantileDiscretizerWriter(this)
 }
 
 @Since("1.6.0")
 object QuantileDiscretizer extends DefaultParamsReadable[QuantileDiscretizer] with Logging {
 
+  private[QuantileDiscretizer]
+  class QuantileDiscretizerWriter(instance: QuantileDiscretizer) extends MLWriter {
+
+    override protected def saveImpl(path: String): Unit = {
+      // SPARK-23377: The default params will be saved and loaded as user-supplied params.
+      // Once `inputCols` is set, the default value of `outputCol` param causes the error
+      // when checking exclusive params. As a temporary to fix it, we skip the default value
+      // of `outputCol` if `inputCols` is set when saving the metadata.
+      // TODO: If we modify the persistence mechanism later to better handle default params,
+      // we can get rid of this.
+      var paramWithoutOutputCol: Option[JValue] = None
+      if (instance.isSet(instance.inputCols)) {
+        val params = instance.extractParamMap().toSeq
+        val jsonParams = params.filter(_.param != instance.outputCol).map { case ParamPair(p, v) =>
+          p.name -> parse(p.jsonEncode(v))
+        }.toList
+        paramWithoutOutputCol = Some(render(jsonParams))
+      }
+      DefaultParamsWriter.saveMetadata(instance, path, sc, paramMap = paramWithoutOutputCol)
+    }
+  }
+
   @Since("1.6.0")
   override def load(path: String): QuantileDiscretizer = super.load(path)
 }

mllib/src/test/scala/org/apache/spark/ml/feature/BucketizerSuite.scala

@@ -172,7 +172,10 @@ class BucketizerSuite extends SparkFunSuite with MLlibTestSparkContext with Defa
       .setInputCol("myInputCol")
       .setOutputCol("myOutputCol")
       .setSplits(Array(0.1, 0.8, 0.9))
-    testDefaultReadWrite(t)
+
+    val bucketizer = testDefaultReadWrite(t)
+    val data = Seq((1.0, 2.0), (10.0, 100.0), (101.0, -1.0)).toDF("myInputCol", "myInputCol2")
+    bucketizer.transform(data)
   }
 
   test("Bucket numeric features") {
@@ -327,7 +330,12 @@ class BucketizerSuite extends SparkFunSuite with MLlibTestSparkContext with Defa
       .setInputCols(Array("myInputCol"))
       .setOutputCols(Array("myOutputCol"))
       .setSplitsArray(Array(Array(0.1, 0.8, 0.9)))
-    testDefaultReadWrite(t)
+
+    val bucketizer = testDefaultReadWrite(t)
+    val data = Seq((1.0, 2.0), (10.0, 100.0), (101.0, -1.0)).toDF("myInputCol", "myInputCol2")
+    bucketizer.transform(data)
+    assert(t.hasDefault(t.outputCol))
+    assert(bucketizer.hasDefault(bucketizer.outputCol))
   }
 
   test("Bucketizer in a pipeline") {

mllib/src/test/scala/org/apache/spark/ml/feature/QuantileDiscretizerSuite.scala

@@ -27,6 +27,8 @@ import org.apache.spark.sql.functions.udf
 class QuantileDiscretizerSuite
   extends SparkFunSuite with MLlibTestSparkContext with DefaultReadWriteTest {
 
+  import testImplicits._
+
   test("Test observed number of buckets and their sizes match expected values") {
     val spark = this.spark
     import spark.implicits._
@@ -132,7 +134,10 @@ class QuantileDiscretizerSuite
       .setInputCol("myInputCol")
       .setOutputCol("myOutputCol")
       .setNumBuckets(6)
-    testDefaultReadWrite(t)
+
+    val readDiscretizer = testDefaultReadWrite(t)
+    val data = sc.parallelize(1 to 100).map(Tuple1.apply).toDF("myInputCol")
+    readDiscretizer.fit(data)
   }
 
   test("Verify resulting model has parent") {
@@ -379,7 +384,12 @@ class QuantileDiscretizerSuite
       .setInputCols(Array("input1", "input2"))
       .setOutputCols(Array("result1", "result2"))
       .setNumBucketsArray(Array(5, 10))
-    testDefaultReadWrite(discretizer)
+
+    val readDiscretizer = testDefaultReadWrite(discretizer)
+    val data = Seq((1.0, 2.0), (2.0, 3.0), (3.0, 4.0)).toDF("input1", "input2")
+    readDiscretizer.fit(data)
+    assert(discretizer.hasDefault(discretizer.outputCol))
+    assert(readDiscretizer.hasDefault(readDiscretizer.outputCol))
   }
 
   test("Multiple Columns: Both inputCol and inputCols are set") {