[SPARK-17507][ML][MLLIB] check weight vector size in ANN

## What changes were proposed in this pull request?

as the TODO described,
check weight vector size and if wrong throw exception.

## How was this patch tested?

existing tests.

Author: WeichenXu <WeichenXu123@outlook.com>

Closes #15060 from WeichenXu123/check_input_weight_size_of_ann.

mllib/src/main/scala/org/apache/spark/ml/ann/Layer.scala

@@ -545,7 +545,9 @@ private[ann] object FeedForwardModel {
    * @return model
    */
   def apply(topology: FeedForwardTopology, weights: Vector): FeedForwardModel = {
-    // TODO: check that weights size is equal to sum of layers sizes
+    val expectedWeightSize = topology.layers.map(_.weightSize).sum
+    require(weights.size == expectedWeightSize,
+      s"Expected weight vector of size ${expectedWeightSize} but got size ${weights.size}.")
     new FeedForwardModel(weights, topology)
   }
 
@@ -559,11 +561,7 @@ private[ann] object FeedForwardModel {
   def apply(topology: FeedForwardTopology, seed: Long = 11L): FeedForwardModel = {
     val layers = topology.layers
     val layerModels = new Array[LayerModel](layers.length)
-    var totalSize = 0
-    for (i <- 0 until topology.layers.length) {
-      totalSize += topology.layers(i).weightSize
-    }
-    val weights = BDV.zeros[Double](totalSize)
+    val weights = BDV.zeros[Double](topology.layers.map(_.weightSize).sum)
     var offset = 0
     val random = new XORShiftRandom(seed)
     for (i <- 0 until layers.length) {