Adding DenseNets (and pretrained models) to the models

[gpleiss]

It would be awesome to have DenseNet imagenet models (and pretrained versions as well) available in the pytorch model zoo.

We could probably just convert the existing pretrained torch models.

Open to a PR?

[colesbury]

Yeah, I think DenseNets would be good. The one thing to keep in mind is that it might be tricky to do the concatenation efficiently