!604 GeneratorDataset column names support string type

Merge pull request !604 from yanghaitao/yht_generatordataset_col_list

mindspore/dataset/engine/datasets.py

@@ -2569,6 +2569,8 @@ class GeneratorDataset(SourceDataset):
                 # Random accessible input is also iterable
                 self.source = (lambda: _iter_fn(source, num_samples))
 
+        if column_names is not None and not isinstance(column_names, list):
+            column_names = [column_names]
         self.column_names = column_names
 
         if column_types is not None: