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/coco.yaml

@@ -1,5 +1,4 @@
 # COCO 2017 dataset http://cocodataset.org
-# Download command: bash yolov5/data/get_coco2017.sh
 # Train command: python train.py --data coco.yaml
 # Default dataset location is next to /yolov5:
 #   /parent_folder
@@ -7,6 +6,9 @@
 #     /yolov5
 
 
+# download command/URL (optional)
+download: bash data/scripts/get_coco.sh
+
 # train and val data as 1) directory: path/images/, 2) file: path/images.txt, or 3) list: [path1/images/, path2/images/]
 train: ../coco/train2017.txt  # 118287 images
 val: ../coco/val2017.txt  # 5000 images