fix dygraph solov2 configs, test=dygraph (#2086)

dygraph/configs/solov2/solov2_r50_fpn_1x_coco.yml

 _BASE_: [
-  '../_base_/datasets/coco_instance.yml',
-  '../_base_/runtime.yml',
+  '../datasets/coco_instance.yml',
+  '../runtime.yml',
   '_base_/solov2_r50_fpn.yml',
   '_base_/optimizer_1x.yml',
   '_base_/solov2_reader.yml',

dygraph/configs/solov2/solov2_r50_fpn_3x_coco.yml

 _BASE_: [
-  '../_base_/datasets/coco_instance.yml',
-  '../_base_/runtime.yml',
+  '../datasets/coco_instance.yml',
+  '../runtime.yml',
   '_base_/solov2_r50_fpn.yml',
   '_base_/optimizer_1x.yml',
   '_base_/solov2_reader.yml',

dygraph/ppdet/modeling/layers.py

@@ -900,6 +900,8 @@ class FCOSBox(object):
         return pred_boxes, pred_scores
 
 
+@register
+@serializable
 class MaskMatrixNMS(object):
     """
     Matrix NMS for multi-class masks.