Support augmentation of single-cell datasets via label projection

[amva13]

Chat: """ Like for example ___ can’t combine the scperturb drug datasets due to batch effects but we could if we had label projection because we’d agree on the control, right? Absolutely! So this functionality should be added to TDC data loaders and data preprocessor. """

This functionality would allow building large-scale single-cell atlases based on a reference dataset (i.e., combining scperturb drug).

Good use case for DSL/MVC