Add pre-trained Fields of The World models

[calebrob6]

Summary

The Fields of The World effort (https://github.com/fieldsoftheworld/ftw-baselines) has released 4 pre-trained models (smp U-Nets with EfficientNet B3 encoders). I've put these on hugging face here, but we still need to integrate into our models API. Previously I think we just have support for encoders, so adding support for these would be new and awesome!