Fix `img2label_paths()` order (#3720)

utils/datasets.py

@@ -348,7 +348,7 @@ class LoadStreams:  # multiple IP or RTSP cameras
 def img2label_paths(img_paths):
     # Define label paths as a function of image paths
     sa, sb = os.sep + 'images' + os.sep, os.sep + 'labels' + os.sep  # /images/, /labels/ substrings
-    return ['txt'.join(x.replace(sa, sb, 1).rsplit(x.split('.')[-1], 1)) for x in img_paths]
+    return [sb.join(x.rsplit(sa, 1)).rsplit('.', 1)[0] + '.txt' for x in img_paths]
 
 
 class LoadImagesAndLabels(Dataset):  # for training/testing