Move torch to a dependency group

[phschoepf]

Since torch/torchvision is still listed in the pyproject.toml, it is currently impossible (aside from manually modifying the dependencies) to use the package without installing torch. Thus, torch should be moved to an optional dependency group.

Unfortunately, this means that torch can't be the default anymore since it won't be installed by default. And there are no "negative entries" for optional dependency groups.

[Paethon]

For me personally, this is only a test until we get feedback from Eng whether it is worth actually removing torch or not. For now, torch is still the preferred and recommended way of doing this in my opinion.

So might be fixed at some time in the future when this is settled. But then we probably also have to improve performance of the non-torch version.