[SPARK-5714][Mllib] Refactor initial step of LDA to remove redundant operations

The `initialState` of LDA performs several RDD operations that looks redundant. This pr tries to simplify these operations.

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #4501 from viirya/sim_lda and squashes the following commits:

4870fe4 [Liang-Chi Hsieh] For comments.
9af1487 [Liang-Chi Hsieh] Refactor initial step of LDA to remove redundant operations.

mllib/src/main/scala/org/apache/spark/mllib/clustering/LDA.scala

@@ -450,34 +450,23 @@ private[clustering] object LDA {
 
     // Create vertices.
     // Initially, we use random soft assignments of tokens to topics (random gamma).
-    val edgesWithGamma: RDD[(Edge[TokenCount], TopicCounts)] =
-      edges.mapPartitionsWithIndex { case (partIndex, partEdges) =>
-        val random = new Random(partIndex + randomSeed)
-        partEdges.map { edge =>
-          // Create a random gamma_{wjk}
-          (edge, normalize(BDV.fill[Double](k)(random.nextDouble()), 1.0))
+    def createVertices(): RDD[(VertexId, TopicCounts)] = {
+      val verticesTMP: RDD[(VertexId, TopicCounts)] =
+        edges.mapPartitionsWithIndex { case (partIndex, partEdges) =>
+          val random = new Random(partIndex + randomSeed)
+          partEdges.flatMap { edge =>
+            val gamma = normalize(BDV.fill[Double](k)(random.nextDouble()), 1.0)
+            val sum = gamma * edge.attr
+            Seq((edge.srcId, sum), (edge.dstId, sum))
+          }
         }
-      }
-    def createVertices(sendToWhere: Edge[TokenCount] => VertexId): RDD[(VertexId, TopicCounts)] = {
-      val verticesTMP: RDD[(VertexId, (TokenCount, TopicCounts))] =
-        edgesWithGamma.map { case (edge, gamma: TopicCounts) =>
-          (sendToWhere(edge), (edge.attr, gamma))
-        }
-      verticesTMP.aggregateByKey(BDV.zeros[Double](k))(
-        (sum, t) => {
-          brzAxpy(t._1, t._2, sum)
-          sum
-        },
-        (sum0, sum1) => {
-          sum0 += sum1
-        }
-      )
+      verticesTMP.reduceByKey(_ + _)
     }
-    val docVertices = createVertices(_.srcId)
-    val termVertices = createVertices(_.dstId)
+
+    val docTermVertices = createVertices()
 
     // Partition such that edges are grouped by document
-    val graph = Graph(docVertices ++ termVertices, edges)
+    val graph = Graph(docTermVertices, edges)
       .partitionBy(PartitionStrategy.EdgePartition1D)
 
     new EMOptimizer(graph, k, vocabSize, docConcentration, topicConcentration, checkpointInterval)