Yes ROCm is for the GPU, but the MI300A also includes 4 clusters of cpus connected by an infinity fabric. Generally this kind of thing is handled by the OS but there is no OS for this product.
AMD has been doing IF-connected CCDs/chiplets for a while now - since Zen 1, released in 2017. All the x86 OSes work fine on each iteration.
Application authors who care about wringing out the last drop of performance need to be mindful about how they manage processes and cache lines on this hardware - as they would on any other architecture
What do you mean no OS? You log into whatever Linux distribution someone put on it, that's one of the better magic tricks from having a collection of x64 cores on the same chip. Or I suppose you roll a unikernel style system if you want to.
