Make clear which subset - 'val' or 'test' (or other)

[RJT1990]

• Need to say in the library if it's validation or test.
• When doing PwC integration, need to also add new field specifying the dataset subset.

For example, I don't have Cityscapes test data, only validation data. But PWC evaluates on the test set, whereas the library will evaluate on the validation set.