Remove torchvision dependency

Literally all we were using torchvision for right now is torchvision.transforms.Compose, and we were not using any other transforms from torchvision.

Compose is trivial and can be re-implemented (having our own version of Compose might be useful as well; tbd).

If we later have a stronger dependency on torchvision we can add it back.

I had thought about doing this before, but it came up again while reworking the transforms interface.