Update webui.py offline mode

[TatsuhiroC]

the original webui.py cannot run without internet connection.

Now, if we already run it once before, it could run offline with models in huggingface cache.

[TatsuhiroC]

SenseVoice user, like me, might use webui feature in any situations, including under internet firewall, Local Area Network, even offline. Therefore, the offline mode is vital.

Please help me if the code could be better. Thanks!!!