Multi-threaded VisDrone and VOC downloads (#7108)

* Multi-threaded VOC download

* Update VOC.yaml

* Update

* Update general.py

* Update general.py

data/coco.yaml

@@ -30,6 +30,7 @@ names: ['person', 'bicycle', 'car', 'motorcycle', 'airplane', 'bus', 'train', 't
 download: |
   from utils.general import download, Path
 
+
   # Download labels
   segments = False  # segment or box labels
   dir = Path(yaml['path'])  # dataset root dir