预处理batch动作设置drop_remainder

跟随训练脚本逻辑，找到预处理函数：

official/vision/image_classification/resnet/imagenet_preprocessing.py，我们直接在该函数内部设置drop_remainder为True，该迁移点完成。

Returns:
  A dataset that can be used for iteration.
"""
drop_remainder=True

if filenames is None:
  filenames = get_filenames(is_training, data_dir)
dataset = tf.data.Dataset.from_tensor_slices(filenames)