[SPARK-17363][ML][MLLIB] fix MultivariantOnlineSummerizer.numNonZeros

## What changes were proposed in this pull request?

fix `MultivariantOnlineSummerizer.numNonZeros` method,
return `nnz` array, instead of  `weightSum` array

## How was this patch tested?

Existing test.

Author: WeichenXu <WeichenXu123@outlook.com>

Closes #14923 from WeichenXu123/fix_MultivariantOnlineSummerizer_numNonZeros.

mllib/src/main/scala/org/apache/spark/mllib/stat/MultivariateOnlineSummarizer.scala

@@ -231,9 +231,9 @@ class MultivariateOnlineSummarizer extends MultivariateStatisticalSummary with S
    */
   @Since("1.1.0")
   override def numNonzeros: Vector = {
-    require(totalWeightSum > 0, s"Nothing has been added to this summarizer.")
+    require(totalCnt > 0, s"Nothing has been added to this summarizer.")
 
-    Vectors.dense(weightSum)
+    Vectors.dense(nnz.map(_.toDouble))
   }
 
   /**

mllib/src/test/scala/org/apache/spark/mllib/stat/MultivariateOnlineSummarizerSuite.scala

@@ -237,7 +237,7 @@ class MultivariateOnlineSummarizerSuite extends SparkFunSuite {
       absTol 1E-10, "mean mismatch")
     assert(summarizer.variance ~== Vectors.dense(Array(0.17657142857, 1.645115714, 2.42057142857))
       absTol 1E-8, "variance mismatch")
-    assert(summarizer.numNonzeros ~== Vectors.dense(Array(0.3, 0.5, 0.4))
+    assert(summarizer.numNonzeros ~== Vectors.dense(Array(3.0, 4.0, 3.0))
       absTol 1E-10, "numNonzeros mismatch")
     assert(summarizer.max ~== Vectors.dense(Array(0.0, 1.7, 1.3)) absTol 1E-10, "max mismatch")
     assert(summarizer.min ~== Vectors.dense(Array(-0.8, -1.2, -1.7)) absTol 1E-10, "min mismatch")