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 @ggerganov (src->buffer to buf_src)

Co-authored-by: Georgi Gerganov [email protected]

• Apply suggestion from @ggerganov (src->buffer to buf_src) v2

Co-authored-by: Georgi Gerganov [email protected]

Co-authored-by: Georgi Gerganov [email protected]

• Apply suggestions from @johannesgaessler code review

Co-authored-by: Johannes Gäßler [email protected]

• Adds single-GPU synchronizations to multi-GPU settings to fix hip backend pipeline parallel bugs.

• Scheduler Hardening: Exclude hip/MUSA from copy_from_host CPU split -> GPU split optimization

• Scheduler Hardening: Re-adding original additional synchronizations for non-async backends

• Adds disclaimer to hip/musa exclusion of copy_from_host. Highlights that it is out of precaution, but that no perf-impact is visible, and that it can be revisited separately anytime.

Co-authored-by: Georgi Gerganov [email protected] Co-authored-by: Johannes Gäßler [email protected]