fix cascade_mask_rcnn export model (PaddlePaddle#4088)

Author: yghstill

configs/cascade_rcnn/README.md

@@ -11,7 +11,6 @@
 | ResNet50-vd-SSLDv2-FPN | Cascade Mask         |    1    |   1x    |     ----     |  44.9  |    39.1    | [下载链接](https://paddledet.bj.bcebos.com/models/cascade_mask_rcnn_r50_vd_fpn_ssld_1x_coco.pdparams) | [配置文件](https://github.com/PaddlePaddle/PaddleDetection/tree/develop/configs/cascade_rcnn/cascade_mask_rcnn_r50_vd_fpn_ssld_1x_coco.yml) |
 | ResNet50-vd-SSLDv2-FPN | Cascade Mask         |    1    |   2x    |     ----     |  45.7  |    39.7    | [下载链接](https://paddledet.bj.bcebos.com/models/cascade_mask_rcnn_r50_vd_fpn_ssld_2x_coco.pdparams) | [配置文件](https://github.com/PaddlePaddle/PaddleDetection/tree/develop/configs/cascade_rcnn/cascade_mask_rcnn_r50_vd_fpn_ssld_2x_coco.yml) |
 
-**注意：** Cascade R-CNN模型精度依赖Paddle develop分支修改，精度复现须使用[每日版本](https://www.paddlepaddle.org.cn/documentation/docs/zh/install/Tables.html#whl-dev)或2.0.1版本(将于2021.03发布)，使用Paddle 2.0.0版本会有少量精度损失。
 
 ## Citations
 ```

ppdet/engine/export_utils.py

@@ -122,7 +122,8 @@ def _dump_infer_config(config, path, image_shape, model):
             format(infer_arch) +
             'Please set TRT_MIN_SUBGRAPH in ppdet/engine/export_utils.py')
         os._exit(0)
-    if 'Mask' in infer_arch:
+    if 'mask_head' in config[config['architecture']] and config[config[
+            'architecture']]['mask_head']:
         infer_cfg['mask'] = True
     label_arch = 'detection_arch'
     if infer_arch in KEYPOINT_ARCH: