[Fix] Fix no revert_sync_batchnorm in image_demo of master branch (#2489)

Fix: https://github.com/open-mmlab/mmsegmentation/issues/2478

demo/image_demo.py

@@ -1,6 +1,8 @@
 # Copyright (c) OpenMMLab. All rights reserved.
 from argparse import ArgumentParser
 
+from mmcv.cnn.utils.sync_bn import revert_sync_batchnorm
+
 from mmseg.apis import inference_segmentor, init_segmentor, show_result_pyplot
 from mmseg.core.evaluation import get_palette
 
@@ -26,6 +28,8 @@ def main():
 
     # build the model from a config file and a checkpoint file
     model = init_segmentor(args.config, args.checkpoint, device=args.device)
+    if args.device == 'cpu':
+        model = revert_sync_batchnorm(model)
     # test a single image
     result = inference_segmentor(model, args.img)
     # show the results