[jvm-packages] Add getNumFeature method (#6075)

* Add getNumFeature to the Java API
* Add getNumFeature to the Scala API
* Add unit tests for getNumFeature

Co-authored-by: Philip Hyunsu Cho <chohyu01@cs.washington.edu>

jvm-packages/xgboost4j/src/main/java/ml/dmlc/xgboost4j/java/Booster.java

@@ -677,6 +677,17 @@ public class Booster implements Serializable, KryoSerializable {
     version += 1;
   }
 
+  /**
+   * Get number of model features.
+   * @return the number of features.
+   * @throws XGBoostError
+   */
+  public long getNumFeature() throws XGBoostError {
+    long[] numFeature = new long[1];
+    XGBoostJNI.checkCall(XGBoostJNI.XGBoosterGetNumFeature(this.handle, numFeature));
+    return numFeature[0];
+  }
+
   /**
    * Internal initialization function.
    * @param cacheMats The cached DMatrix.

jvm-packages/xgboost4j/src/main/java/ml/dmlc/xgboost4j/java/XGBoostJNI.java

@@ -120,6 +120,7 @@ class XGBoostJNI {
   public final static native int XGBoosterSetAttr(long handle, String key, String value);
   public final static native int XGBoosterLoadRabitCheckpoint(long handle, int[] out_version);
   public final static native int XGBoosterSaveRabitCheckpoint(long handle);
+  public final static native int XGBoosterGetNumFeature(long handle, long[] feature);
 
   // rabit functions
   public final static native int RabitInit(String[] args);

jvm-packages/xgboost4j/src/main/scala/ml/dmlc/xgboost4j/scala/Booster.scala

@@ -291,6 +291,14 @@ class Booster private[xgboost4j](private[xgboost4j] var booster: JBooster)
         .asScala.mapValues(_.doubleValue).toSeq: _*)
   }
 
+  /**
+    * Get the number of model features.
+    *
+    * @return number of features
+    */
+  @throws(classOf[XGBoostError])
+  def getNumFeature: Long = booster.getNumFeature
+
   def getVersion: Int = booster.getVersion
 
   def toByteArray: Array[Byte] = {