Fixing typing issue.

Author: datumbox

torchvision/datasets/stl10.py

@@ -182,4 +182,6 @@ def __load_folds(self, folds: Optional[int]) -> None:
         with open(path_to_folds, 'r') as f:
             str_idx = f.read().splitlines()[folds]
             list_idx = np.fromstring(str_idx, dtype=np.uint8, sep=' ')
-            self.data, self.labels = self.data[list_idx, :, :, :], self.labels[list_idx]
+            self.data = self.data[list_idx, :, :, :]
+            if self.labels is not None:
+                self.labels = self.labels[list_idx]