fix faster rcnn eval (#1892)

fluid/PaddleCV/rcnn/eval_coco_map.py

@@ -111,8 +111,10 @@ def eval():
     assert len(dts_res) > 0, "The number of valid bbox detected is zero.\n \
         Please use reasonable model and check input data."
 
-    assert len(segms_res) > 0, "The number of valid mask detected is zero.\n \
-        Please use reasonable model and check input data.."
+    if cfg.MASK_ON:
+        assert len(
+            segms_res) > 0, "The number of valid mask detected is zero.\n \
+            Please use reasonable model and check input data.."
 
     with open("detection_bbox_result.json", 'w') as outfile:
         json.dump(dts_res, outfile)