Implement on-demand model swapping for multiple models

- Add model_backend_types dict to track backend for each model
- Update set_default_model to accept backend_type parameter
- Modify get_model_for_request to swap models on-demand when in ondemand mode
- Unload current model from VRAM and load new model when request arrives for different model
- Respect --backend flag when loading models on-demand
- Only activates when no --loadall or --loadswap flag is specified