ggml : add alloc_buffer_n to buffer type interface (llama/23671)

* 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

* cont : fix `cur_buf_size` init after flushing a buffer

* ggml : add TODO tag for shared buffer split logic

Assisted-by: pi:llama.cpp/DeepSeek-V4-Flash-Vision-Exp

* tests : add alloc_buffer_n coverage

Assisted-by: pi:llama.cpp/DeepSeek-V4-Flash-Vision-Exp

* cont : fix compile warnings

* tests : add descriptions for alloc_buffer_n tests

Assisted-by: pi:llama.cpp/Qwen3.8-27B

* ggml : address review comments on alloc_buffer_n

- restore GGML_LOG_ERROR on buffer alloc / tensor init failure in the
  default impl (name the failing tensor)
- check the malloc result and drop the _impl indirection in
  ggml_backend_alloc_ctx_tensors_from_buft
- remove comments that restate the code
- fix the TAG_ALLOC_SHARED_BUFFER_SPLIT typo

Assisted-by: pi:llama.cpp/Qwen3.8-27B

* ggml : add get_alloc_size_n to buffer type interface

- Add ggml_backend_buft_get_alloc_size_n public API
- Add optional get_alloc_size_n callback to ggml_backend_buffer_type_i
- Share tensor->buffer planning between alloc_buffer_n default and get_alloc_size_n default
- Replace unchecked realloc with std::vector in alloc_buffer_n default
- Make ggml_backend_alloc_ctx_tensors_from_buft_size use the new API
- Add test-alloc coverage for get_alloc_size_n

Assisted-by: pi:llama.cpp/DeepSeek-V4-Flash-Vision-Exp

* cont : report malloc failure

ggml/include/ggml-alloc.h

@@ -75,9 +75,10 @@ GGML_API size_t ggml_gallocr_get_buffer_size(ggml_gallocr_t galloc, int buffer_i
 
 // Utils
 // Create a buffer and allocate all the tensors in a ggml_context
-// ggml_backend_alloc_ctx_tensors_from_buft_size returns the size of the buffer that would be allocated by ggml_backend_alloc_ctx_tensors_from_buft
-// ggml_backend_alloc_ctx_tensors_from_buft returns NULL on failure or if all tensors in ctx are already allocated or zero-sized
+
+// returns the size of the buffer that would be allocated by ggml_backend_alloc_ctx_tensors_from_buft. returns 0 on failure
 GGML_API size_t                       ggml_backend_alloc_ctx_tensors_from_buft_size(struct ggml_context * ctx, ggml_backend_buffer_type_t buft);
+// returns NULL on failure or if all tensors in ctx are already allocated or zero-sized
 GGML_API struct ggml_backend_buffer * ggml_backend_alloc_ctx_tensors_from_buft(struct ggml_context * ctx, ggml_backend_buffer_type_t buft);
 GGML_API struct ggml_backend_buffer * ggml_backend_alloc_ctx_tensors(struct ggml_context * ctx, ggml_backend_t backend);
 

ggml/include/ggml-backend.h

@@ -34,13 +34,15 @@ extern "C" {
     // Backend buffer type
     //
 
-    GGML_API const char *          ggml_backend_buft_name          (ggml_backend_buffer_type_t buft);
-    GGML_API ggml_backend_buffer_t ggml_backend_buft_alloc_buffer  (ggml_backend_buffer_type_t buft, size_t size);
-    GGML_API size_t                ggml_backend_buft_get_alignment (ggml_backend_buffer_type_t buft);
-    GGML_API size_t                ggml_backend_buft_get_max_size  (ggml_backend_buffer_type_t buft);
-    GGML_API size_t                ggml_backend_buft_get_alloc_size(ggml_backend_buffer_type_t buft, const struct ggml_tensor * tensor);
-    GGML_API bool                  ggml_backend_buft_is_host       (ggml_backend_buffer_type_t buft);
-    GGML_API ggml_backend_dev_t    ggml_backend_buft_get_device    (ggml_backend_buffer_type_t buft);
+    GGML_API const char *          ggml_backend_buft_name            (ggml_backend_buffer_type_t buft);
+    GGML_API ggml_backend_buffer_t ggml_backend_buft_alloc_buffer    (ggml_backend_buffer_type_t buft, size_t size);
+    GGML_API ggml_backend_buffer_t ggml_backend_buft_alloc_buffer_n  (ggml_backend_buffer_type_t buft, struct ggml_tensor ** tensors, int n_tensors);
+    GGML_API size_t                ggml_backend_buft_get_alignment   (ggml_backend_buffer_type_t buft);
+    GGML_API size_t                ggml_backend_buft_get_max_size    (ggml_backend_buffer_type_t buft);
+    GGML_API size_t                ggml_backend_buft_get_alloc_size  (ggml_backend_buffer_type_t buft, const struct ggml_tensor * tensor);
+    GGML_API size_t                ggml_backend_buft_get_alloc_size_n(ggml_backend_buffer_type_t buft, struct ggml_tensor ** tensors, int n_tensors);
+    GGML_API bool                  ggml_backend_buft_is_host         (ggml_backend_buffer_type_t buft);
+    GGML_API ggml_backend_dev_t    ggml_backend_buft_get_device      (ggml_backend_buffer_type_t buft);
 
     //
     // Backend buffer