common: refactor model handling (#24980)

* common: refactor models handling

* remote preset

* cont

* rm skip_download option

* missing header

* fix plan.model_files

* fix --offline case

* move hf_plan to download

* refactor

* rm redundant curr_ex, add comments

* adapt

app/download.cpp

@@ -38,7 +38,8 @@ int llama_download(int argc, char ** argv) {
     }
 
     try {
-        common_params_handle_models(params, LLAMA_EXAMPLE_DOWNLOAD, {});
+        common_models_handler handler = common_models_handler_init(params, LLAMA_EXAMPLE_DOWNLOAD);
+        common_models_handler_apply(handler, params);
     } catch (const std::exception & e) {
         fprintf(stderr, "error: %s\n", e.what());
         return 1;