cv2.imread(img, -1) for IMREAD_UNCHANGED (ultralytics#3379)

* Update datasets.py

* comment

Co-authored-by: Glenn Jocher <glenn.jocher@ultralytics.com>
(cherry picked from commit 21a9607)

utils/datasets.py

@@ -181,7 +181,7 @@ def __next__(self):
         else:
             # Read image
             self.count += 1
-            img0 = cv2.imread(path)  # BGR
+            img0 = cv2.imread(path, -1)  # BGR (-1 is IMREAD_UNCHANGED)
             assert img0 is not None, 'Image Not Found ' + path
             print(f'image {self.count}/{self.nf} {path}: ', end='')