Another big tensor fix b9820

sched : reintroduce less synchronizations during split compute (#20793) CUDA: Improve performance via less synchronizations between token (#17795) Adds CPU-to-CUDA copy capability to ggml_backend_cuda_cpy_tensor_async() Adds function to relax sync requirements between input copies on supported backends (CUDA for now) Exchanges synchronous copy with async copy function. Adds macro guards to allow compilation in non-CUDA builds Reworked backend detection in ggml-backend.cpp to avoid linking conflicts Relax requirement of checks in async CUDA copies from backend and buffer type to just buffer type, to avoid linking issues Minor cleanup Makes opt-in to relax use of explicit syncs more general. Backends like vulkan which require a synchronization between HtoD copies and graph execution could also adopt this change now. Reintroduces stricter check for CPU->CUDA backend async copy via GGML_DEVICE_TYPE_CPU. Corrects initialization of ggml_backend_sync_mode in ggml_backend_sched_split initialization Simplifies synchronizations to adhere to saaasg pattern. Apply suggestion from u/ggerganov (src->buffer to buf_src)