Add __all__ to mnist.py

Author: wangkuiyi

python/paddle/v2/dataset/mnist.py

@@ -2,17 +2,16 @@
 import subprocess
 import numpy
 
+__all__ = ['train', 'test']
+
 URL_PREFIX = 'http://yann.lecun.com/exdb/mnist/'
 
 TEST_IMAGE_URL = URL_PREFIX + 't10k-images-idx3-ubyte.gz'
 TEST_IMAGE_MD5 = '25e3cc63507ef6e98d5dc541e8672bb6'
-
 TEST_LABEL_URL = URL_PREFIX + 't10k-labels-idx1-ubyte.gz'
 TEST_LABEL_MD5 = '4e9511fe019b2189026bd0421ba7b688'
-
 TRAIN_IMAGE_URL = URL_PREFIX + 'train-images-idx3-ubyte.gz'
 TRAIN_IMAGE_MD5 = 'f68b3c2dcbeaaa9fbdd348bbdeb94873'
-
 TRAIN_LABEL_URL = URL_PREFIX + 'train-labels-idx1-ubyte.gz'
 TRAIN_LABEL_MD5 = 'd53e105ee54ea40749a09fcbcd1e9432'