Multi-GPU on same device

[AlexCheema]

Not a problem on Macs since they only have 1 GPU. On other machines, we default to tinygrad. Right now we pick the DEFAULT device.

The desired behaviour needs to be scoped out here. The simplest would be require the user (we can also provide a helper script) to run multiple instances of exo on the same device. We'd need to change how discovery works to support multiple instances of exo on the same device.

[rallisf1]

When #173 is merged one can simply run a docker container per GPU.