HUB dataset previews to JPEG (#5627)

@kalenmike per our convo yesterday.

utils/datasets.py

@@ -965,7 +965,7 @@ def dataset_stats(path='coco128.yaml', autodownload=False, verbose=False, profil
             r = max_dim / max(im.height, im.width)  # ratio
             if r < 1.0:  # image too large
                 im = im.resize((int(im.width * r), int(im.height * r)))
-            im.save(f_new, quality=75)  # save
+            im.save(f_new, 'JPEG', quality=75, optimize=True)  # save
         except Exception as e:  # use OpenCV
             print(f'WARNING: HUB ops PIL failure {f}: {e}')
             im = cv2.imread(f)