This likely requires a fair bit of work.

Since Metal and CUDA are both C++ based it would be great if field implementations (and other functionality) could be shared between the CUDA and Metal code. Some issues might be the the address space keywords that Metal uses i.e. "constant" (which are being used for constants in field implementations).

Also the first gpu-poly version was written for my M1 Mac. M1 has a unified memory architecture so the memory doesn't have to be moved to and from the GPU. This will no longer be the case if CUDA support is added. Might be worth creating a new Buffer type that abstracts away CPU<->GPU memory movement from the library.