Multi-threaded VisDrone and VOC downloads (#7108)

* Multi-threaded VOC download

* Update VOC.yaml

* Update

* Update general.py

* Update general.py

data/VOC.yaml

@@ -62,7 +62,7 @@ download: |
   urls = [url + 'VOCtrainval_06-Nov-2007.zip',  # 446MB, 5012 images
           url + 'VOCtest_06-Nov-2007.zip',  # 438MB, 4953 images
           url + 'VOCtrainval_11-May-2012.zip']  # 1.95GB, 17126 images
-  download(urls, dir=dir / 'images', delete=False)
+  download(urls, dir=dir / 'images', delete=False, threads=3)
 
   # Convert
   path = dir / f'images/VOCdevkit'