`load_demo_single` called with wrong argument type

[MrBMRBLX]

In fastchat/serve/gradio_web_server.py, the function load_demo_single expects a Context object but load_demo passes a plain list.

def load_demo_single(context: Context, query_params):
    models = context.text_models  # expects Context

def load_demo(url_params, request: gr.Request):
    return load_demo_single(models, url_params)  # passes list

This would cause an AttributeError when running in reload mode.