Split multimodal group into vision and audio (#419)

## Summary

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Turns the `guidellm[multimodal]` extras group into `guidellm[audio]` and
`guidellm[vision]`.

---

- [x] "I certify that all code in this PR is my own, except as noted
below."

## Use of AI

- [ ] Includes AI-assisted code completion
- [ ] Includes code generated by an AI application
- [ ] Includes AI-generated tests (NOTE: AI written tests should have a
docstring that includes `## WRITTEN BY AI ##`)

Author: sjmonson