[ITensorGPU] [ENHANCEMENT] Add Github Action to test loading ITensorGPU

[mtfishman]

Add a Github Action to test loading ITensorGPU (and the new NDTensorCUDA extension) on systems without CUDA installed, since we want that to work like CUDA.jl does. It could just be a script that executes using ITensorGPU.

If needed, CUDA.functional() can be used as a barrier to make sure code doesn't execute at load time that relies on CUDA functionality.

If this isn't handled properly, it can cause issues for registering new versions of ITensorGPU: JuliaRegistries/General#82516

See for example:
https://cuda.juliagpu.org/stable/installation/conditional/#Scenario-2:-GPU-is-optional
7120090