Update utils/datasets.py to support .webp files (ultralytics#2174)

Simply added 'webp' as an image format to the img_formats array so that webp image files can be used as training data.

utils/datasets.py

@@ -25,7 +25,7 @@
 
 # Parameters
 help_url = 'https://github.com/ultralytics/yolov5/wiki/Train-Custom-Data'
-img_formats = ['bmp', 'jpg', 'jpeg', 'png', 'tif', 'tiff', 'dng']  # acceptable image suffixes
+img_formats = ['bmp', 'jpg', 'jpeg', 'png', 'tif', 'tiff', 'dng', 'webp']  # acceptable image suffixes
 vid_formats = ['mov', 'avi', 'mp4', 'mpg', 'mpeg', 'm4v', 'wmv', 'mkv']  # acceptable video suffixes
 logger = logging.getLogger(__name__)