ggml : implement a spellcheck model (xfspell, t5-spellchecker, etc)

Apple had recently announced a new transformer-based keyboard auto-correct and prediction.

xfspell seems to be an existing model that tried doing it, so why not investigate if it can be ported to GGML. If anyone know other models for predictive keyboard or auto correct, please drop your suggestions here.

Perhaps this may even be a good test case for on-device QLoRA fine-tuning.

High quality predictive keyboards and auto-correct in pure C++ can be a useful thing for open-source mobile operating systems like Ubuntu Touch and privacy-focused Android ROMs, because traditionally, such proposals got rejected because of excessive dependencies for ML inference.