Update voc dataset url (#30450)

* update voc url

python/paddle/vision/datasets/voc2012.py

@@ -25,8 +25,7 @@ from paddle.dataset.common import _check_exists_and_download
 
 __all__ = ["VOC2012"]
 
-VOC_URL = 'http://host.robots.ox.ac.uk/pascal/VOC/voc2012/\
-VOCtrainval_11-May-2012.tar'
+VOC_URL = 'https://dataset.bj.bcebos.com/voc/VOCtrainval_11-May-2012.tar'
 
 VOC_MD5 = '6cd6e144f989b92b3379bac3b3de84fd'
 SET_FILE = 'VOCdevkit/VOC2012/ImageSets/Segmentation/{}.txt'
@@ -42,6 +41,9 @@ class VOC2012(Dataset):
     """
     Implementation of `VOC2012 <http://host.robots.ox.ac.uk/pascal/VOC/voc2012/>`_ dataset
 
+    To speed up the download, we put the data on https://dataset.bj.bcebos.com/voc/VOCtrainval_11-May-2012.tar. 
+    Original data can get from http://host.robots.ox.ac.uk/pascal/VOC/voc2012/VOCtrainval_11-May-2012.tar.
+
     Args:
         data_file(str): path to data file, can be set None if
             :attr:`download` is True. Default None