If you do the transformation to black and white on the fly, you can just do it in your training loop or add it as an additional transform.
That said, it’s very easy and maybe instructive to just implement your own dataset loading pairs of images, too (using the transforms of torchvision). Really, all you need to do is load the images, convert them to tensors and return a pair of them. PyTorch will take care of the rest.
Best regards
Thomas