Making root parameter optional for Vision Dataset (#8124)

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

Author: zshn25

torchvision/datasets/vision.py

@@ -12,7 +12,7 @@ class VisionDataset(data.Dataset):
     It is necessary to override the ``__getitem__`` and ``__len__`` method.
 
     Args:
-        root (string): Root directory of dataset.
+        root (string, optional): Root directory of dataset. Only used for `__repr__`.
         transforms (callable, optional): A function/transforms that takes in
             an image and a label and returns the transformed versions of both.
         transform (callable, optional): A function/transform that  takes in an PIL image
@@ -29,7 +29,7 @@ class VisionDataset(data.Dataset):
 
     def __init__(
         self,
-        root: str,
+        root: Optional[str] = None,
         transforms: Optional[Callable] = None,
         transform: Optional[Callable] = None,
         target_transform: Optional[Callable] = None,