Move Bibtex entries for models and datasets

Instead of in the global README.md, it's better to move them into each module. This keeps it local, and makes it easier to maintain given the many references we use on a per-method/layer/etc. basis. For example, in [Rank-1 BNNs](https://github.com/google/uncertainty-baselines/blob/ff19fb8665168fc8163875ca25f100fc2a12fe2c/baselines/cifar/rank1_bnn.py#L22).