Handle empty defined splits rather than raise error.

If the split is defined as empty in the metadata file, then treat the split as empty.

PiperOrigin-RevId: 649076154

Author: The TensorFlow Datasets Authors

tensorflow_datasets/core/splits.py

@@ -476,10 +476,11 @@ def _file_instructions_for_split(
 ) -> List[shard_utils.FileInstruction]:
   """Returns the file instructions from the given instruction applied to the given split info."""
   if not split_info.num_examples:
-    raise ValueError(
-        "Shard empty. This might means that dataset hasn't been generated "
-        'yet and info not restored from GCS, or that legacy dataset is used.'
+    logging.warning(
+        'Split %s has no examples. Skipping file instructions.',
+        split_info.name,
     )
+    return []
   to = split_info.num_examples if instruction.to is None else instruction.to
   return shard_utils.get_file_instructions(
       from_=instruction.from_ or 0,

tensorflow_datasets/core/splits_test.py

@@ -627,20 +627,18 @@ def test_touching_boundaries(self):
     self.assertEqual(files, [])
 
   def test_missing_shard_lengths(self):
-    with self.assertRaisesWithPredicateMatch(ValueError, 'Shard empty.'):
-      filename_template = _filename_template(
-          split='train', dataset_name='mnist'
-      )
-      split_infos = [
-          splits.SplitInfo(
-              name='train',
-              shard_lengths=[],
-              num_bytes=0,
-              filename_template=filename_template,
-          ),
-      ]
-      splits_dict = splits.SplitDict(split_infos=split_infos)
-      _ = splits_dict['train'].file_instructions
+    filename_template = _filename_template(split='train', dataset_name='mnist')
+    split_infos = [
+        splits.SplitInfo(
+            name='train',
+            shard_lengths=[],
+            num_bytes=0,
+            filename_template=filename_template,
+        ),
+    ]
+    splits_dict = splits.SplitDict(split_infos=split_infos)
+    files = splits_dict['train'].file_instructions
+    self.assertEqual(files, [])
 
 
 if __name__ == '__main__':