allow for running w/ multiple gpus and provide an example

[lowrollr]

          And i need use mult gpus, I don't see something like pmap in code. Need a example that how to train use mult gpus.

Originally posted by @Nightbringers in https://github.com/lowrollr/turbozero/issues/8#issuecomment-1925039705

[lowrollr]

added with #12