Fix for ImageNet dataset (#858)

* Fix wrong doc string

* bug fix

torchvision/datasets/imagenet.py

@@ -45,7 +45,7 @@ class ImageNet(ImageFolder):
         classes (list): List of the class names.
         class_to_idx (dict): Dict with items (class_name, class_index).
         wnids (list): List of the WordNet IDs.
-        class_to_idx (dict): Dict with items (wordnet_id, wordnet_id_index).
+        wnid_to_idx (dict): Dict with items (wordnet_id, class_index).
         imgs (list): List of (image path, class_index) tuples
         targets (list): The class_index value for each image in the dataset
     """
@@ -156,7 +156,7 @@ def download_and_extract_tar(url, download_root, extract_root=None, filename=Non
                              md5=None, **kwargs):
     download_root = os.path.expanduser(download_root)
     if extract_root is None:
-        extract_root = extract_root
+        extract_root = download_root
     if filename is None:
         filename = os.path.basename(url)