3D Computation Clarity in unet_3d, resnet_3d, and transform_3d

[chenxinhua]

I have a question: unet_3d, resnet_3d, and transform_3d only deal with dimensional transformations, yet there is no indication anywhere that 3D computation is necessary.

[lixunsong]

In stage2, motion module will be inserted into the 3d unet. This will perform temporal self-attention, which differs from 2d unet. So we keep them seperately.