[SPARK-15608][ML][EXAMPLES][DOC] add examples and documents of ml.isotonic regression

## What changes were proposed in this pull request?

add ml doc for ml isotonic regression
add scala example for ml isotonic regression
add java example for ml isotonic regression
add python example for ml isotonic regression

modify scala example for mllib isotonic regression
modify java example for mllib isotonic regression
modify python example for mllib isotonic regression

add data/mllib/sample_isotonic_regression_libsvm_data.txt
delete data/mllib/sample_isotonic_regression_data.txt
## How was this patch tested?

N/A

Author: WeichenXu <WeichenXu123@outlook.com>

Closes #13381 from WeichenXu123/add_isotonic_regression_doc.

2 files changed, 67 insertions, 4 deletions

diff --git a/examples/src/main/scala/org/apache/spark/examples/ml/IsotonicRegressionExample.scala b/examples/src/main/scala/org/apache/spark/examples/ml/IsotonicRegressionExample.scala
new file mode 100644
index 0000000000..7c5d3f2341
--- /dev/null
+++ b/examples/src/main/scala/org/apache/spark/examples/ml/IsotonicRegressionExample.scala
@@ -0,0 +1,62 @@
+/*
+ * 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.
+ */
+
+// scalastyle:off println
+package org.apache.spark.examples.ml
+
+// $example on$
+import org.apache.spark.ml.regression.IsotonicRegression
+// $example off$
+import org.apache.spark.sql.SparkSession
+
+/**
+ * An example demonstrating Isotonic Regression.
+ * Run with
+ * {{{
+ * bin/run-example ml.IsotonicRegressionExample
+ * }}}
+ */
+object IsotonicRegressionExample {
+
+  def main(args: Array[String]): Unit = {
+
+    // Creates a SparkSession.
+    val spark = SparkSession
+      .builder
+      .appName(s"${this.getClass.getSimpleName}")
+      .getOrCreate()
+
+    // $example on$
+    // Loads data.
+    val dataset = spark.read.format("libsvm")
+      .load("data/mllib/sample_isotonic_regression_libsvm_data.txt")
+
+    // Trains an isotonic regression model.
+    val ir = new IsotonicRegression()
+    val model = ir.fit(dataset)
+
+    println(s"Boundaries in increasing order: ${model.boundaries}")
+    println(s"Predictions associated with the boundaries: ${model.predictions}")
+
+    // Makes predictions.
+    model.transform(dataset).show()
+    // $example off$
+
+    spark.stop()
+  }
+}
+// scalastyle:on println
diff --git a/examples/src/main/scala/org/apache/spark/examples/mllib/IsotonicRegressionExample.scala b/examples/src/main/scala/org/apache/spark/examples/mllib/IsotonicRegressionExample.scala
index c4336639d7..e5dea129c1 100644
--- a/examples/src/main/scala/org/apache/spark/examples/mllib/IsotonicRegressionExample.scala
+++ b/examples/src/main/scala/org/apache/spark/examples/mllib/IsotonicRegressionExample.scala
@@ -21,6 +21,7 @@ package org.apache.spark.examples.mllib
 import org.apache.spark.{SparkConf, SparkContext}
 // $example on$
 import org.apache.spark.mllib.regression.{IsotonicRegression, IsotonicRegressionModel}
+import org.apache.spark.mllib.util.MLUtils
 // $example off$
 
 object IsotonicRegressionExample {
@@ -30,12 +31,12 @@ object IsotonicRegressionExample {
     val conf = new SparkConf().setAppName("IsotonicRegressionExample")
     val sc = new SparkContext(conf)
     // $example on$
-    val data = sc.textFile("data/mllib/sample_isotonic_regression_data.txt")
+    val data = MLUtils.loadLibSVMFile(sc,
+      "data/mllib/sample_isotonic_regression_libsvm_data.txt").cache()
 
     // Create label, feature, weight tuples from input data with weight set to default value 1.0.
-    val parsedData = data.map { line =>
-      val parts = line.split(',').map(_.toDouble)
-      (parts(0), parts(1), 1.0)
+    val parsedData = data.map { labeledPoint =>
+      (labeledPoint.label, labeledPoint.features(0), 1.0)
     }
 
     // Split data into training (60%) and test (40%) sets.