Fix stylecheck in io (#3126)

Summary:
`Dict` is not used. Fix styecheck by removing the import of `Dict`.

Pull Request resolved: https://github.com/pytorch/audio/pull/3126

Reviewed By: mthrok

Differential Revision: D43699410

Pulled By: nateanl

fbshipit-source-id: 8d6b5335124903453387c488f96f297d6fe3c819

torchaudio/io/_compat.py

 import os
 import sys
-from typing import BinaryIO, Dict, Optional, Tuple, Union
+from typing import BinaryIO, Optional, Tuple, Union
 
 import torch
 import torchaudio