CUDA now allows concurrent streams per split for multi-GPU.

The official source reports this update: b10782: CUDA: Allow concurrent streams per split for multi-GPU. CUDA: Allow concurrent streams per split for multi-GPU ( #28198 ) CUDA: Allow CUDA optimization per split for multi-GPU. Previous guard caused multi-GPU to skip the graph optimization. The graph is already split per device and the optimization doesnt run over the whole model but once per split, and thus should be allowed. However, the CUDA event ggml_cuda_concurrent_event belongs to whichever GPU was "current" when created. If the pass ran while GPU 0 was current, it would stick and during event creation for the second GPU it would land on GPU 0.