ggml : add `alloc_buffer_n` to buffer type interface
Add alloc_buffer_n method to ggml_backend_buffer_type_i
interface, with a public API ggml_backend_buft_alloc_buffer_n.
- Default implementation in ggml-backend.cpp handles multi-buffer
splitting and tensor allocation via ggml_tallocr
- Meta buffer type provides custom implementation that creates
per-device sub-contexts and delegates to simple buffer types
- ggml_backend_alloc_ctx_tensors_from_buft now collects tensors
into a list and delegates to the new API
- Remove temporary ggml_backend_meta_alloc_ctx_tensors_from_buft
- Add NULL alloc_buffer_n to all existing buffer type
interfaces (cpu, metal, openvino, hexagon, webgpu, zdnn, virtgpu, repack)
Assisted-by: llama.cpp:local pi
