Folder datasets should share same labels mapping across collaborators

[Efrat1]

Describe the bug In FolderDataset we use LabelMapper to map each label string (sample's folder name) to int. This mapping is common for all data sources but not across collaborators.

To Reproduce One collaborator has samples of a set of labels (folders) while other collaborator has a different set.

Expected behavior Should have either a common configuration with decided mapping shared among all collaborators, or, in all collaborators, use an agreed deterministic 1 to 1 conversion from label string to integer.