[SPARK-13036][SPARK-13318][SPARK-13319] Add save/load for feature.py

Add save/load for feature.py. Meanwhile, add save/load for `ElementwiseProduct` in Scala side and fix a bug of missing `setDefault` in `VectorSlicer` and `StopWordsRemover`.

In this PR I ignore the `RFormula` and `RFormulaModel` because its Scala implementation is pending in https://github.com/apache/spark/pull/9884. I'll add them in this PR if https://github.com/apache/spark/pull/9884 gets merged first. Or add a follow-up JIRA for `RFormula`.

Author: Xusen Yin <yinxusen@gmail.com>

Closes #11203 from yinxusen/SPARK-13036.

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

@@ -17,10 +17,10 @@
 
 package org.apache.spark.ml.feature
 
-import org.apache.spark.annotation.Experimental
+import org.apache.spark.annotation.{Experimental, Since}
 import org.apache.spark.ml.UnaryTransformer
 import org.apache.spark.ml.param.Param
-import org.apache.spark.ml.util.Identifiable
+import org.apache.spark.ml.util.{DefaultParamsReadable, DefaultParamsWritable, Identifiable}
 import org.apache.spark.mllib.feature
 import org.apache.spark.mllib.linalg.{Vector, VectorUDT}
 import org.apache.spark.sql.types.DataType
@@ -33,7 +33,7 @@ import org.apache.spark.sql.types.DataType
  */
 @Experimental
 class ElementwiseProduct(override val uid: String)
-  extends UnaryTransformer[Vector, Vector, ElementwiseProduct] {
+  extends UnaryTransformer[Vector, Vector, ElementwiseProduct] with DefaultParamsWritable {
 
   def this() = this(Identifiable.randomUID("elemProd"))
 
@@ -57,3 +57,10 @@ class ElementwiseProduct(override val uid: String)
 
   override protected def outputDataType: DataType = new VectorUDT()
 }
+
+@Since("2.0.0")
+object ElementwiseProduct extends DefaultParamsReadable[ElementwiseProduct] {
+
+  @Since("2.0.0")
+  override def load(path: String): ElementwiseProduct = super.load(path)
+}

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

+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.ml.feature
+
+import org.apache.spark.SparkFunSuite
+import org.apache.spark.ml.util.DefaultReadWriteTest
+import org.apache.spark.mllib.linalg.Vectors
+import org.apache.spark.mllib.util.MLlibTestSparkContext
+
+class ElementwiseProductSuite
+  extends SparkFunSuite with MLlibTestSparkContext with DefaultReadWriteTest {
+
+  test("read/write") {
+    val ep = new ElementwiseProduct()
+      .setInputCol("myInputCol")
+      .setOutputCol("myOutputCol")
+      .setScalingVec(Vectors.dense(0.1, 0.2))
+    testDefaultReadWrite(ep)
+  }
+}