Fix infinite reload loop when gradio mounted as a sub application

[freddyaboulton]

Description

Fixes #2427

The fix is to set dev mode to false in when gradio is mounted as sub application. My thinking is that if you're mounting gradio as a sub app and want hot reloading, you can just launch fast api in reload mode yourself.

Checklist:

• I have performed a self-review of my own code
• I have added a short summary of my change to the CHANGELOG.md
• My code follows the style guidelines of this project
• I have commented my code in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

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[github-actions]

All the demos for this PR have been deployed at https://huggingface.co/spaces/gradio-pr-deploys/pr-2477-all-demos

[abidlabs]

Thanks for fixing this @freddyaboulton! The logic makes sense and I've confirmed that it works locally.