vulkan: prefer host-visible memory buffers on UMA devices (ggml-org#22930)

* implement UMA host-visible memory

* update based on 0cc4m's suggestion

Author: winstonma

ggml/src/ggml-vulkan/ggml-vulkan.cpp

@@ -3080,8 +3080,10 @@ static vk_buffer ggml_vk_create_buffer_device(vk_device& device, size_t size) {
             buf = ggml_vk_create_buffer(device, size, {vk::MemoryPropertyFlagBits::eHostVisible | vk::MemoryPropertyFlagBits::eHostCoherent,
                                                        vk::MemoryPropertyFlagBits::eDeviceLocal});
         } else if (device->uma) {
-            // Fall back to host memory type
-            buf = ggml_vk_create_buffer(device, size, {vk::MemoryPropertyFlagBits::eDeviceLocal,
+            // On UMA, prefer host-visible memory so direct tensor borrowing works.
+            // If unavailable, fall back to device-local memory.
+            buf = ggml_vk_create_buffer(device, size, {vk::MemoryPropertyFlagBits::eDeviceLocal | vk::MemoryPropertyFlagBits::eHostVisible | vk::MemoryPropertyFlagBits::eHostCoherent,
+                                                       vk::MemoryPropertyFlagBits::eDeviceLocal,
                                                        vk::MemoryPropertyFlagBits::eHostVisible | vk::MemoryPropertyFlagBits::eHostCoherent});
         } else if (device->disable_host_visible_vidmem) {
             if (device->allow_sysmem_fallback) {