Intel GPU/XPU support

[talmo]

PyTorch has support for Intel GPUs as of v2.5.

There's a good range of hardware options that are supported: https://docs.pytorch.org/docs/stable/notes/get_start_xpu.html

Relevant: https://www.intel.com/content/www/us/en/docs/oneapi/optimization-guide-gpu/2025-0/intel-xe-gpu-architecture.html

We should double check to make sure that all of our code still works with the "xpu" device backend and expose some easy controls for this.

It looks like there may also be some dependencies that need to be tweaked.

In case it requires a different PyTorch build entirely, we may want to just make the PyTorch dependencies optional and group them under different dependency groups.

[eberrigan]

Related to #247