data-autodownload background tasks (#2034)

data/scripts/get_voc.sh

@@ -17,9 +17,11 @@ url=https://github.com/ultralytics/yolov5/releases/download/v1.0/
 f1=VOCtrainval_06-Nov-2007.zip # 446MB, 5012 images
 f2=VOCtest_06-Nov-2007.zip     # 438MB, 4953 images
 f3=VOCtrainval_11-May-2012.zip # 1.95GB, 17126 images
-for f in $f1 $f2 $f3; do
-  echo 'Downloading' $url$f ' ...' && curl -L $url$f -o $f && unzip -q $f -d $d && rm $f # download, unzip, remove
+for f in $f3 $f2 $f1; do
+  echo 'Downloading' $url$f '...' && curl -L $url$f -o $f # download, (unzip, remove in background)
+  unzip -q $f -d $d && rm $f &
 done
+wait # finish background tasks
 
 end=$(date +%s)
 runtime=$((end - start))