[Enhancement] Support inherit cityscapes dataset (#750)

* rewrite init function

* init function parameters

mmseg/datasets/cityscapes.py

@@ -29,11 +29,12 @@ class CityscapesDataset(CustomDataset):
                [255, 0, 0], [0, 0, 142], [0, 0, 70], [0, 60, 100],
                [0, 80, 100], [0, 0, 230], [119, 11, 32]]
 
-    def __init__(self, **kwargs):
+    def __init__(self,
+                 img_suffix='_leftImg8bit.png',
+                 seg_map_suffix='_gtFine_labelTrainIds.png',
+                 **kwargs):
         super(CityscapesDataset, self).__init__(
-            img_suffix='_leftImg8bit.png',
-            seg_map_suffix='_gtFine_labelTrainIds.png',
-            **kwargs)
+            img_suffix=img_suffix, seg_map_suffix=seg_map_suffix, **kwargs)
 
     @staticmethod
     def _convert_to_label_id(result):