Improve ImageNet documentation (#7503)

Co-authored-by: Nicolas Hug <contact@nicolas-hug.com>
Co-authored-by: Nicolas Hug <nh.nicolas.hug@gmail.com>

torchvision/datasets/imagenet.py

@@ -21,6 +21,12 @@ META_FILE = "meta.bin"
 class ImageNet(ImageFolder):
     """`ImageNet <http://image-net.org/>`_ 2012 Classification Dataset.
 
+    .. note::
+        Before using this class, it is required to download ImageNet 2012 dataset from
+        `here <https://image-net.org/challenges/LSVRC/2012/2012-downloads.php>`_ and
+        place the files ``ILSVRC2012_devkit_t12.tar.gz`` and ``ILSVRC2012_img_train.tar``
+        or ``ILSVRC2012_img_val.tar`` based on ``split`` in the root directory.
+
     Args:
         root (string): Root directory of the ImageNet Dataset.
         split (string, optional): The dataset split, supports ``train``, or ``val``.