Add DenseNet

[divyashreepathihalli]

This task is part of the KerasHub project ( Consolidating all of KerasCV and KerasNLP models in to one place)
Models from KerasCV will be added to KerasNLP in KerasNLP infrstructure style. The development is now under the keras-hub branch in KerasNLP - https://github.com/keras-team/keras-nlp/tree/keras-hub

Steps to complete this Task

• Source code :https://github.com/keras-team/keras-cv/tree/master/keras_cv/src/models/backbones/densenet
• Destination : A model folder here https://github.com/keras-team/keras-nlp/tree/keras-hub/keras_nlp/src/models
• Example PR : Add VGG16 and VGG19 backbone #1737, please follow the style from here and take a look at the comments on the PR for any design clarifications
• copy the model over
• If the model has multiple versions with small changes in architecture, they should be added as one model and the configuration should be handled in the args
• use KerasNLP test routines to update the tests
• for reformatting the code run shell/api_gen.sh, shell/format.sh and shell/lint.sh
• The backbone will be added as a model, any downstream task model will be under the same model folder.

Add either @divyashreepathihalli, @mattdangerw or @SamanehSaadat for review of any PR.