[Docs] demo.py only gives result of one single image

[ztianlin]

Branch

main branch https://mmdetection3d.readthedocs.io/en/latest/

📚 The doc issue

When executing demo.py for occupancy visualization, I only get one result (correspond to the first image in image folder) and press N/D in open3d window will led to "no more images".

Suggest a potential alternative/fix

No response