Generate grid from flow field, used for bilinear warping. See grid_gen.
Extracting patches from 4D tensors can be simply implemented by torch.nn.functional.unfold but unsupported for 5D tensors. And this can be achieved by tensor.unfold, which is quite different from torch.nn.functional.unfold, and call it at W, H, D dimension sequentially. See extract_3d_patches.