[SPARK-24300][ML] change the way to set seed in ml.cluster.LDASuite.generateLDAData

## What changes were proposed in this pull request?

Using different RNG in all different partitions.

## How was this patch tested?

manually

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

Author: Lu WANG <lu.wang@databricks.com>

Closes #21492 from ludatabricks/SPARK-24300.

Author: lu-wang-dl

mllib/src/test/scala/org/apache/spark/ml/clustering/LDASuite.scala

@@ -34,9 +34,8 @@ object LDASuite {
       vocabSize: Int): DataFrame = {
     val avgWC = 1  // average instances of each word in a doc
     val sc = spark.sparkContext
-    val rng = new java.util.Random()
-    rng.setSeed(1)
     val rdd = sc.parallelize(1 to rows).map { i =>
+      val rng = new java.util.Random(i)
       Vectors.dense(Array.fill(vocabSize)(rng.nextInt(2 * avgWC).toDouble))
     }.map(v => new TestRow(v))
     spark.createDataFrame(rdd)