!2775 Set dataset size in generator when input has a length

Merge pull request !2775 from ZiruiWu/better_get_dataset_size_in_generator

mindspore/dataset/engine/datasets.py

@@ -3157,6 +3157,9 @@ class GeneratorDataset(MappableDataset):
                 self.column_names.append(col["name"])
                 self.column_types.append(DataType(col["type"]))
 
+        if source is not None and hasattr(source, "__len__"):
+            self._dataset_size = len(source)
+
     def get_args(self):
         args = super().get_args()
         args["source"] = self.source
@@ -3177,6 +3180,7 @@ class GeneratorDataset(MappableDataset):
             return self._dataset_size
         if self._dataset_size is None:
             return None
+
         return min(rows_from_sampler, self._dataset_size)
 
     # manually set dataset_size as a temporary solution.