Change the way NDArrayIter handle the last batch (apache#12285)

* 1. move the shuffle to the reset 2. modify the roll_over behavior accordingly

* refactor the concat part

* refactor the code

* implement unit test for last_batch_handle

* refactor the getdata part

* add docstring and refine the code according to linter

* 1. add test case for NDArrayIter_h5py 2. refactor the implementation

* update contributions doc

* fix wording

* update doc for roll_over

* 1. add test for second iteration of roll_over 2. add shuffle test case

* fix some wording and refine the variables naming

* move utility function to new file

* move utility function to io_utils.py

* change shuffle function name to avoid redefining name

* make io as a module

* rename the utility functions

* disable wildcard-import

CONTRIBUTORS.md

@@ -178,3 +178,4 @@ List of Contributors
 * [Aaron Markham](https://github.com/aaronmarkham)
 * [Sam Skalicky](https://github.com/samskalicky)
 * [Per Goncalves da Silva](https://github.com/perdasilva)
+* [Cheng-Che Lee](https://github.com/stu1130)

python/mxnet/io/__init__.py

@@ -0,0 +1,29 @@
+#!/usr/bin/env python
+
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+
+# coding: utf-8
+# pylint: disable=wildcard-import
+""" Data iterators for common data formats and utility functions."""
+from __future__ import absolute_import
+
+from . import io
+from .io import *
+
+from . import utils
+from .utils import *