[higgs_audio_v2] Fix: use config.num_codebooks in audio labels tensor (Simple Fix!) (#48560)
* [higgs_audio_v2] Fix: use config.num_codebooks in audio labels tensor
Fix - `HiggsAudioV2ForConditionalGeneration.forward` allocated the expanded audio label tensor with a hardcoded last dimension of 8.
* [higgs_audio_v2] Test: add audio label config test