fix: unload engine before updating

Author: sangjanai

engine/controllers/engines.cc

@@ -379,6 +379,8 @@ void Engines::UpdateEngine(
           metadata = (*exist_engine).metadata;
         }
 
+        (void) engine_service_->UnloadEngine(engine);
+
         auto upd_res =
             engine_service_->UpsertEngine(engine, type, api_key, url, version,
                                           "all-platforms", status, metadata);

engine/services/engine_service.cc

@@ -906,10 +906,10 @@ cpp::result<void, std::string> EngineService::UnloadEngine(
     auto unload_opts = EngineI::EngineUnloadOption{};
     e->Unload(unload_opts);
     delete e;
-    engines_.erase(ne);
   } else {
     delete std::get<RemoteEngineI*>(engines_[ne].engine);
   }
+  engines_.erase(ne);
 
   CTL_DBG("Engine unloaded: " + ne);
   return {};

engine/services/model_service.cc

@@ -1255,6 +1255,8 @@ cpp::result<std::optional<std::string>, std::string>
 ModelService::MayFallbackToCpu(const std::string& model_path, int ngl,
                                int ctx_len, int n_batch, int n_ubatch,
                                const std::string& kv_cache_type) {
+  // TODO(sang) temporary disable this function
+  return std::nullopt;
   assert(hw_service_);
   auto hw_info = hw_service_->GetHardwareInfo();
   assert(!!engine_svc_);