[FX] _generate_dummy_input supports audio-classification models for labels
For FX:

- Adds support for audio-classification models for label generation in `_generate_dummy_input`
- Adds a flag, `FX_DEBUG_MODE` to control what's being printed during tracing, this will save the user from seeing a lot of benign warnings, while still providing the possibility to have those while developing.
_The documentation is not available anymore as the PR was closed or merged._