Fix bug that subsampled_streams is modified during iteration

This extracts the keys from subsampled_streams so it can be modified in the loop

video2dataset/workers/download_worker.py

@@ -234,7 +234,7 @@ def data_generator():
                     )
                     subsampled_streams, metas, error_message = broadcast_subsampler(streams, meta)
 
-                    for modality in subsampled_streams:
+                    for modality in list(subsampled_streams.keys()):
                         for modality_subsampler in self.subsamplers[modality]:
                             subsampled_streams, metas, error_message = modality_subsampler(subsampled_streams, metas)