Backwards compatible cache version checks (#3730)

utils/datasets.py

@@ -390,7 +390,7 @@ class LoadImagesAndLabels(Dataset):  # for training/testing
         cache_path = (p if p.is_file() else Path(self.label_files[0]).parent).with_suffix('.cache')  # cached labels
         if cache_path.is_file():
             cache, exists = torch.load(cache_path), True  # load
-            if cache['version'] != 0.3 or cache['hash'] != get_hash(self.label_files + self.img_files):
+            if cache.get('version') != 0.3 or cache.get('hash') != get_hash(self.label_files + self.img_files):
                 cache, exists = self.cache_labels(cache_path, prefix), False  # re-cache
         else:
             cache, exists = self.cache_labels(cache_path, prefix), False  # cache