Not doubting what is officially grammatically correct, but that still sounds really weird to me. Like with sports teams I would only ever say "The Patriots are a good team" or "New England is a good team". Not "The Patriots is a good team".
In any event, the authors definitely chose that title as a callback to the well known paper "Attention is all you need", which introduced Transformers. So that probably influenced their decision to use "is" instead of "are".
