Add Gradio Demo

[freddyaboulton]

Checklist

• Read CONTRIBUTING.md, and accept the CLA by including the provided snippet. We will not accept PR without this.
• Run pre-commit hook.
• If you changed Rust code, run cargo check, cargo clippy, cargo test.

I, @freddyaboulton, confirm that I have read and understood the terms of the CLA of Kyutai-labs, as outlined in the repository's CONTRIBUTING.md, and I agree to be bound by these terms.

PR Description

Adds a Gradio demo to let developers built web apps with moshi entirely with python

[LaurentMazare]

Thanks!