llama: use FA + max. GPU layers by default (#15434)

* llama: use max. GPU layers by default, auto -fa

* ggml-backend: abort instead of segfault

src/llama-model.cpp

@@ -18994,7 +18994,7 @@ llama_model_params llama_model_default_params() {
     llama_model_params result = {
         /*.devices                     =*/ nullptr,
         /*.tensor_buft_overrides       =*/ nullptr,
-        /*.n_gpu_layers                =*/ 0,
+        /*.n_gpu_layers                =*/ 999,
         /*.split_mode                  =*/ LLAMA_SPLIT_MODE_LAYER,
         /*.main_gpu                    =*/ 0,
         /*.tensor_split                =*/ nullptr,
@@ -19008,11 +19008,6 @@ llama_model_params llama_model_default_params() {
         /*.use_extra_bufts             =*/ true,
     };
 
-#ifdef GGML_USE_METAL
-    // note: we usually have plenty of VRAM, so by default offload all layers to the GPU
-    result.n_gpu_layers = 999;
-#endif
-
     return result;
 }