Fix gradio web server demo error

[tsunghan-wu]

Why are these changes needed?

A quick fix for the single-model Gradio web server. The original code was passing a list to the Context object, which caused an AttributeError. This PR corrects that issue by ensuring the proper Context object is passed.

Related issue number (if applicable):

#3595

Checks:

• I've run format.sh to lint the changes in this PR.
• I've included any necessary doc changes. (No doc changes required.)
• I've ensured that the relevant tests are passing (if applicable).

Test:

Follow the instructions here, and the error no longer occurs after running the controller, model worker, and Gradio worker, then opening the browser.