[SPARK-18698][ML] Adding public constructor that takes uid for IndexToString

## What changes were proposed in this pull request?

Based on SPARK-18698, this adds a public constructor that takes a UID for IndexToString.  Other transforms have similar constructors.

## How was this patch tested?

A unit test was added to verify the new functionality.

Author: Ilya Matiach <ilmat@microsoft.com>

Closes #16436 from imatiach-msft/ilmat/fix-indextostring.

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

@@ -255,7 +255,7 @@ object StringIndexerModel extends MLReadable[StringIndexerModel] {
  * @see `StringIndexer` for converting strings into indices
  */
 @Since("1.5.0")
-class IndexToString private[ml] (@Since("1.5.0") override val uid: String)
+class IndexToString @Since("2.2.0") (@Since("1.5.0") override val uid: String)
   extends Transformer with HasInputCol with HasOutputCol with DefaultParamsWritable {
 
   @Since("1.5.0")

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

@@ -20,7 +20,7 @@ package org.apache.spark.ml.feature
 import org.apache.spark.{SparkException, SparkFunSuite}
 import org.apache.spark.ml.attribute.{Attribute, NominalAttribute}
 import org.apache.spark.ml.param.ParamsSuite
-import org.apache.spark.ml.util.{DefaultReadWriteTest, MLTestingUtils}
+import org.apache.spark.ml.util.{DefaultReadWriteTest, Identifiable, MLTestingUtils}
 import org.apache.spark.mllib.util.MLlibTestSparkContext
 import org.apache.spark.sql.Row
 import org.apache.spark.sql.functions.col
@@ -219,6 +219,12 @@ class StringIndexerSuite
     testDefaultReadWrite(t)
   }
 
+  test("SPARK 18698: construct IndexToString with custom uid") {
+    val uid = "customUID"
+    val t = new IndexToString(uid)
+    assert(t.uid == uid)
+  }
+
   test("StringIndexer metadata") {
     val data = Seq((0, "a"), (1, "b"), (2, "c"), (3, "a"), (4, "a"), (5, "c"))
     val df = data.toDF("id", "label")