Musicgen ONNX export (text-conditional only) (#1779)
* WIP but need to work on encodec first
* musicgen onnx export
* better logs
* add tests
* rename audio_encoder_decode.onnx to encodec_decode.onnx
* fix num heads in pkv
* nits
* add build_delay_pattern_mask
* fix wrong hidden_size for cross attention pkv
* fix tests
* update doc
