force the user to set number of workers

jvm-packages/xgboost4j-flink/src/main/scala/ml/dmlc/xgboost4j/scala/flink/XGBoost.scala

@@ -72,10 +72,7 @@ object XGBoost {
     */
   def loadModelFromHadoopFile(modelPath: String) : XGBoostModel = {
     new XGBoostModel(
-      XGBoostScala.loadModel(
-        FileSystem
-          .get(new Configuration)
-          .open(new Path(modelPath))))
+      XGBoostScala.loadModel(FileSystem.get(new Configuration).open(new Path(modelPath))))
   }
 
   /**
@@ -85,11 +82,9 @@ object XGBoost {
     * @param params The parameters to XGBoost.
     * @param round Number of rounds to train.
     */
-  def train(
-      dtrain: DataSet[LabeledVector],
-      params: Map[String, Any],
-      round: Int): XGBoostModel = {
-    val tracker = new RabitTracker(dtrain.getExecutionEnvironment.getParallelism)
+  def train(dtrain: DataSet[LabeledVector], params: Map[String, Any], round: Int, nWorkers: Int):
+      XGBoostModel = {
+    val tracker = new RabitTracker(nWorkers)
     if (tracker.start()) {
       dtrain
         .mapPartition(new MapFunction(params, round, tracker.getWorkerEnvs))