Update ace.py

ace.py

@@ -79,7 +79,7 @@ class MockupDataset(datasets.GeneratorBasedBuilder):
     # method parameters are unpacked from `gen_kwargs` as given in `_split_generators`
     def _generate_examples(self, split):
         for i in range(self.data_config['size']):
-            yield key, {
+            yield i, {
                 "x": [0]*self.data_config['length'],
                 "y": [1]*self.data_config['length']
             }