Define XPU backend scope

[ehartford]

Define the initial QuixiCore XPU backend scope.

Platform:

• Intel GPU / oneAPI / SYCL / Level Zero

Initial targets:

• intel_arc
• intel_data_center_gpu
• future_xpu

Repository:

• QuixiAI/quixicore-xpu

Note:

• XPU remains separate from Gaudi because the compiler stack, runtime system, programming model, and user workflow are different.