handle possibly empty batch

src/axolotl/monkeypatch/data/batch_dataset_fetcher.py

@@ -9,6 +9,9 @@ from torch.utils.data._utils.worker import _worker_loop
 
 class _MapDatasetFetcher(_BaseDatasetFetcher):
     def fetch(self, possibly_batched_index):
+        if not possibly_batched_index:
+            return self.collate_fn([])
+
         if isinstance(possibly_batched_index[0], list):
             data = [None for i in possibly_batched_index]
             for i, possibly_batched_index_ in enumerate(possibly_batched_index):