Update gradio_web_server.py

[lsy641]

trafficstars

bug fix: do moderation_filter only if do_moderation == True

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• [x] I've run format.sh to lint the changes in this PR.
• [x] I've included any doc changes needed.
• [x] I've made sure the relevant tests are passing (if applicable).

[lsy641]

To fix this issue #3345