fix: map mmapped weights through Metal buffers instead of CPU buffers (#2037)

src/model_loader.h

@@ -20,6 +20,8 @@ struct ModelFileData {
     std::vector<TensorStorage> tensors;
     std::shared_ptr<MmapWrapper> mmapped;
     std::shared_ptr<struct ggml_backend_buffer> mmbuffer;
+    // mmapped wrapped by devices that can use host memory in place (buffer_from_host_ptr)
+    std::map<ggml_backend_dev_t, std::shared_ptr<struct ggml_backend_buffer>> device_mmbuffers;
     bool is_zip;
 };
 
@@ -120,7 +122,9 @@ public:
     void process_model_files(bool enable_mmap = false, bool writable_mmap = true);
     std::vector<MmapTensorStore> mmap_tensors(std::map<std::string, ggml_tensor*>& tensors,
                                               std::set<std::string> ignore_tensors = {},
-                                              bool writable                        = true);
+                                              bool writable                        = true,
+                                              ggml_backend_dev_t device            = nullptr);
+    std::vector<ggml_backend_buffer_t> get_device_mmap_buffers() const;
     bool load_tensors(on_new_tensor_cb_t on_new_tensor_cb,
                       bool use_mmap                                    = false,
                       const std::set<std::string>* target_tensor_names = nullptr,