[dev] fix infer bug of s2anet (#3080)

* fix infer bug of s2anet

configs/dota/README.md

@@ -97,7 +97,7 @@ python3.7 tools/infer.py -c configs/dota/s2anet_1x_dota.yml -o weights=./weights
 
 ## 预测部署
 
-Paddle中`multiclass_nms`算子的输入支持四边形输入，因此部署时可以不不需要依赖旋转框IOU计算算子。
+Paddle中`multiclass_nms`算子的输入支持四边形输入，因此部署时可以不需要依赖旋转框IOU计算算子。
 
 ```bash
 # 预测

ppdet/engine/export_utils.py

@@ -112,6 +112,11 @@ def _dump_infer_config(config, path, image_shape, model):
             config['TestReader'], config['TestDataset'], config['metric'],
             label_arch, image_shape)
 
+    if infer_arch == 'S2ANet':
+        # TODO: move background to num_classes
+        if infer_cfg['label_list'][0] != 'background':
+            infer_cfg['label_list'].insert(0, 'background')
+
     yaml.dump(infer_cfg, open(path, 'w'))
     logger.info("Export inference config file to {}".format(os.path.join(path)))
     return image_shape