standalone ICON and GradICON losses for registration network training

[BailiangJ]

Hi @HastingsGreer ,

Thanks a lot for sharing the code.

I have created the standalone ICON and GradICON losses with Pytorch, which can be directly used in general registration network training. Modification would be much simpler to plug in and play with them.

Relevant to issue #79

Double check is needed to see if there is any error in the implementation. :)

Cheers,
Bailiang

[BailiangJ]

Since the flow field in the official code is normalized to [-1,1] while the implementation here is the displacement in the original resolution, epsilon = torch.randn_like(forward_flow) * (1.0 / self.image_size[-1]) in line 313,382 and delta in GradICONLoss might need to be changed accordingly.