ENH: Add pre-trained model checkpoints

[NickleDave]

• a blocker for this might be ENH: Make pre-trained model functionality more flexible, clearer #669 ENH: Add model_path option to config for eval + predict, use in place of checkpoint_path, spect_scaler, etc. #672 Declare/formalize a model file format #673
• use torch.hub https://pytorch.org/docs/stable/hub.html#loading-models-from-hub
• we probably want a separate vak-model-zoo repo and for checkpoints
• for bonus points, make the training of those models reproducible from that repo