data-autodownload background tasks (#2034)

data/scripts/get_coco.sh

@@ -20,5 +20,7 @@ f1='train2017.zip' # 19G, 118k images
 f2='val2017.zip'   # 1G, 5k images
 f3='test2017.zip'  # 7G, 41k images (optional)
 for f in $f1 $f2; do
-  echo 'Downloading' $url$f ' ...' && curl -L $url$f -o $f && unzip -q $f -d $d && rm $f # download, unzip, remove
+  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