Add get_coco128.sh for downloading the coco128 dataset (#3047)

data/scripts/get_coco128.sh

+#!/bin/bash
+# COCO128 dataset https://www.kaggle.com/ultralytics/coco128
+# Download command: bash data/scripts/get_coco128.sh
+# Train command: python train.py --data coco128.yaml
+# Default dataset location is next to /yolov5:
+#   /parent_folder
+#     /coco128
+#     /yolov5
+
+# Download/unzip images and labels
+d='../' # unzip directory
+url=https://github.com/ultralytics/yolov5/releases/download/v1.0/
+f='coco128.zip' # or 'coco2017labels-segments.zip', 68 MB
+echo 'Downloading' $url$f ' ...'
+curl -L $url$f -o $f && unzip -q $f -d $d && rm $f & # download, unzip, remove in background
+
+wait # finish background tasks