ggml-backend : add GGML_BACKEND_DEVICE_TYPE_IGPU device type (#15797)
* ggml-backend : add GGML_BACKEND_DEVICE_TYPE_IGPU device type
ggml-backend : add device id to device props
llama : only use iGPU devices if there are no GPU devices
llama : do not use multiple devices from different backends with the same device id