add webp format available in torch datasets folder (#736)

Author: rcourivaud

torchvision/datasets/folder.py

@@ -151,7 +151,7 @@ def __repr__(self):
         return fmt_str
 
 
-IMG_EXTENSIONS = ['.jpg', '.jpeg', '.png', '.ppm', '.bmp', '.pgm', '.tif', '.tiff']
+IMG_EXTENSIONS = ['.jpg', '.jpeg', '.png', '.ppm', '.bmp', '.pgm', '.tif', '.tiff', 'webp']
 
 
 def pil_loader(path):