Remove unused import os in datasets (#4879)

* remove unused imports

* update lsun

torchvision/datasets/cifar.py

-import os
 import os.path
 import pickle
 from typing import Any, Callable, Optional, Tuple

torchvision/datasets/coco.py

-import os
 import os.path
 from typing import Any, Callable, Optional, Tuple, List
 

torchvision/datasets/lsun.py

 import io
-import os
 import os.path
 import pickle
 import string

torchvision/datasets/semeion.py

-import os
 import os.path
 from typing import Any, Callable, Optional, Tuple
 

torchvision/datasets/stl10.py

-import os
 import os.path
 from typing import Any, Callable, Optional, Tuple
 

torchvision/datasets/svhn.py

-import os
 import os.path
 from typing import Any, Callable, Optional, Tuple