remove NDArray.apply because it is really confusing

scala-package/core/src/main/scala/ml/dmlc/mxnet/NDArray.scala

@@ -322,12 +322,16 @@ class NDArray(val handle: NDArrayHandle, val writable: Boolean = true) {
    *
    * @return a sliced NDArray that shares memory with current one.
    */
-  private def _slice(start: Int, stop: Int): NDArray = {
+  def slice(start: Int, stop: Int): NDArray = {
     val sliceHandle = new NDArrayHandle()
     checkCall(_LIB.mxNDArraySlice(handle, start, stop, sliceHandle))
     new NDArray(handle = sliceHandle, writable = this.writable)
   }
 
+  def slice(start: Int): NDArray = {
+    slice(start, shape(0))
+  }
+
   /**
    * Block until all pending writes operations on current NDArray are finished.
    * This function will return when all the pending writes to the current
@@ -336,8 +340,6 @@ class NDArray(val handle: NDArrayHandle, val writable: Boolean = true) {
    */
   def waitToRead(): Unit = ???
 
-  def apply(sliceStart: Int, sliceEnd: Int): NDArray = _slice(sliceStart, sliceEnd)
-
   /**
    * Get context of current NDArray.
    * @return The context of current NDArray.