Fix imports in CIFAR-PI baselines

[gortizji]

I have spotted a few issues in the new baselines in baselines/privileged_information/cifar_pi:

1. The local imports in the baselines (e.g., utils.py, ood_utils.py, pi_utils.py) are not recognised when you run as suggested in the README.md.
2. The new datasets are not listed in uncertainty_baselines/datasets/datasets.py and uncertainty_baselines/datasets/__init__.py.
3. cifar10n and cifar100n are not allowed options of FLAGS.dataset.
4. The new augmix flags are not declared anywhere.

Issues 2., 3., and 4. should be fixed by this PR, but the local imports in 1. need to be fixed. This will probably require some restructuring of the baselines directory, either duplicating the utils files inside cifar_pi or working out the relative imports explicitly.