Dataset autodownload feature addition (#685)

* initial commit

* move download scripts into data/scripts

* new check_dataset() function in general.py

* move check_dataset() out of with context

* Update general.py

* DDP update

* Update general.py

data/voc.yaml

@@ -1,5 +1,4 @@
 # PASCAL VOC dataset http://host.robots.ox.ac.uk/pascal/VOC/
-# Download command: bash ./data/get_voc.sh
 # Train command: python train.py --data voc.yaml
 # Default dataset location is next to /yolov5:
 #   /parent_folder
@@ -7,6 +6,9 @@
 #     /yolov5
 
 
+# download command/URL (optional)
+download: bash data/scripts/get_voc.sh
+
 # train and val data as 1) directory: path/images/, 2) file: path/images.txt, or 3) list: [path1/images/, path2/images/]
 train: ../VOC/images/train/  # 16551 images
 val: ../VOC/images/val/  # 4952 images