Add an optional backend interface method free_scratch (with a public
ggml_backend_free_scratch wrapper) that frees transient/scratch device
memory a backend holds outside of any allocated buffer, keeping the
backend usable - the scratch is reallocated lazily on the next compute.
Implement it for the Vulkan backend (ggml_backend_vk_free_scratch): free
the prealloc_x/y/split_k/add_rms_partials and sync_staging device buffers
and reset their sizes, so an idle/cold model does not hold the vision or
matmul compute preallocations in VRAM. All other backends leave the hook
null (no-op).
Assisted-by: Claude
Lumpiasty
a8dc3ebddb