Dispatcher for model architectures.

This module handles model versioning, ensuring that trained models can be loaded even if the library's default architecture changes.

Adding a new architecture

1. Create a new module e.g. BinClass.Model.NewArch in lib/bin_class/model/new_arch.ex.
2. Implement build/2 in that module.
3. Add a new clause to BinClass.Model.build/3:

   def build(:new_arch, vocab_size, opts), do: BinClass.Model.NewArch.build(vocab_size, opts)

4. Update @model_version in BinClass.Trainer to :new_arch.