Pass a list instead of an iterator

This should fix the dataset viewer.

Iterators are stateful, so if you use the dataset several times or load it in streaming you can end up with an empty iterator in `files`

NIH-Chest-X-ray-dataset.py

@@ -195,14 +195,14 @@ class ChestXray14(datasets.GeneratorBasedBuilder):
 		    datasets.SplitGenerator(
 			name=datasets.Split.TRAIN,
 			gen_kwargs={
-				"files": iter(train_files)
+				"files": train_files
 			}
 
 		    ),
 		    datasets.SplitGenerator(
 			name=datasets.Split.TEST,
 			gen_kwargs={
-				"files": iter(test_files)
+				"files": test_files
 			}
 		    )
 		]