[ONNX export] Musicgen for text-to-audio

[xenova]

Feature request

Musicgen was recently added to 🤗 Transformers (model doc) and it would be great to be able to export those models to ONNX with Optimum.

Motivation

This will allow me to support music generation models in Transformers.js

Your contribution

I will integrate into transformers.js once available in optimum.