I can import mmpose successfully. But when I run the demo " python demo/body3d_multiview_detect_and_regress_img_demo.py \ configs/body/3d_kpt_mview_rgb_img/voxelpose ...

I guess this might be a version problem. #1734 was fixed after the release of MMPose v0.29. So if you install MMPose as a third-party package, you might miss it.