[FastSpeech2] Investigate fine-tuning of models with different pitch/energy bins

The pitch and energy bins are determined based on the `stats.json` of a particular dataset, which will change during fine-tuning. Currently the bins will change and the model has to adapt and learn how they have changed, but we should maybe consider some other fine-tuning strategies and pay attention to how energy/pitch losses perform in different pretrain/finetuning set-ups