[SPARK-25268][GRAPHX] run Parallel Personalized PageRank throws seria…

…lization Exception

## What changes were proposed in this pull request?
mapValues in scala is currently not serializable. To avoid the serialization issue while running pageRank, we need to use map instead of mapValues.

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

Closes #22271 from shahidki31/master_latest.

Authored-by: Shahid <shahidki31@gmail.com>
Signed-off-by: Joseph K. Bradley <joseph@databricks.com>

graphx/src/main/scala/org/apache/spark/graphx/lib/PageRank.scala

@@ -198,9 +198,11 @@ object PageRank extends Logging {
 
     val zero = Vectors.sparse(sources.size, List()).asBreeze
     // map of vid -> vector where for each vid, the _position of vid in source_ is set to 1.0
-    val sourcesInitMap = sources.zipWithIndex.toMap.mapValues { i =>
-      Vectors.sparse(sources.size, Array(i), Array(1.0)).asBreeze
-    }
+    val sourcesInitMap = sources.zipWithIndex.map { case (vid, i) =>
+      val v = Vectors.sparse(sources.size, Array(i), Array(1.0)).asBreeze
+      (vid, v)
+    }.toMap
+
     val sc = graph.vertices.sparkContext
     val sourcesInitMapBC = sc.broadcast(sourcesInitMap)
     // Initialize the PageRank graph with each edge attribute having