Add NFNet support

[jxtps]

Introduced in https://arxiv.org/abs/2102.06171 discussed in https://news.ycombinator.com/item?id=26133798 and https://www.youtube.com/watch?v=rNkHjZtH0RQ with code on https://github.com/deepmind/deepmind-research/tree/master/nfnets (in JAX), Normalizer-Free Networks are an interesting new development in deep learning for image processing: they figured out a way to do away with BatchNorm, with a host of alleged upsides (see paper for details).

It would be very interesting to have NFNets be a part of the classification_models, thanks!