Make Graph{,Impl,Ops} serializable to work around capture

graphx/src/main/scala/org/apache/spark/graphx/Graph.scala

@@ -20,7 +20,7 @@ import org.apache.spark.storage.StorageLevel
  * @tparam VD the vertex attribute type
  * @tparam ED the edge attribute type
  */
-abstract class Graph[VD: ClassTag, ED: ClassTag] {
+abstract class Graph[VD: ClassTag, ED: ClassTag] protected () extends Serializable {
 
   /**
    * An RDD containing the vertices and their associated attributes.

graphx/src/main/scala/org/apache/spark/graphx/GraphOps.scala

@@ -14,7 +14,7 @@ import org.apache.spark.rdd.RDD
  * @tparam VD the vertex attribute type
  * @tparam ED the edge attribute type
  */
-class GraphOps[VD: ClassTag, ED: ClassTag](graph: Graph[VD, ED]) {
+class GraphOps[VD: ClassTag, ED: ClassTag](graph: Graph[VD, ED]) extends Serializable {
 
   /** The number of edges in the graph. */
   lazy val numEdges: Long = graph.edges.count()

graphx/src/main/scala/org/apache/spark/graphx/impl/GraphImpl.scala

@@ -32,7 +32,7 @@ class GraphImpl[VD: ClassTag, ED: ClassTag] protected (
     @transient val replicatedVertexView: ReplicatedVertexView[VD])
   extends Graph[VD, ED] with Serializable {
 
-  /** Default construct is provided to support serialization */
+  /** Default constructor is provided to support serialization */
   protected def this() = this(null, null, null, null)
 
   /** Return a RDD that brings edges together with their source and destination vertices. */