[FEAT]: Metal support

[weikengchen]

Description

Recommend Ingo to explore Metal acceleration as well. It seems to become something that requires a skillful player to work on it.

Motivation

Metal means the GPU acceleration available on Apple M1/M2/M3 chips. Due to its unique UMA architecture, these chips appear to be performing very well. RISC Zero has this datasheet (https://dev.risczero.com/datasheet.pdf) that talks about the performance with different hardware, where Metal really stands out.

Although Metal is not widely used in cloud computing, a lot of developers have it, people who are building ZK applications on the phone (yes, we have one, and they will reach out!) do need Metal.

Metal is not the easiest to play with, not just because it has less popularity than CUDA, but Apple may be changing this or that. It really needs a professional team to help navigate this area.

For example, RISC Zero has encountered an issue recently about Metal support from Apple due to new limitations on the number of temporary registers allowed in a given function.
risc0/risc0#999