bug fixed

[danialht]

In SyntheticallyCorruptedImageFolderDataset constructor, the default value for corruptions_dict was a mutable, which can result in unexpected behavior. Classic Python gotcha, but now is fixed.

Furthermore, non-zero corruption_probability with an empty dict, e.g. the default value, would lead to a ValueError the first time that the corruption is triggered in __get_item__ because np.random.choice would raise ValueError when passed an empty list. Instead for this situation we can fail faster in the constructor than to fail lazily in __get_item__.