llama : allow using iGPUs with --device (#15951)

* llama : allow using iGPUs with --device

* mtmd : allow iGPU

* rpc-server : allow iGPU

tools/mtmd/clip.cpp

@@ -406,6 +406,7 @@ struct clip_ctx {
             }
             if (!backend) {
                 backend = ggml_backend_init_by_type(GGML_BACKEND_DEVICE_TYPE_GPU, nullptr);
+                backend = backend ? backend : ggml_backend_init_by_type(GGML_BACKEND_DEVICE_TYPE_IGPU, nullptr);
             }
         }