Add possibility to apply transforms also to submatrices

[einbandi]

In t-SNE, for example, the probability submatrix (p_ij) needs to be normalized to make it comparable to the batch-wise probability matrix (q_ij).

I can think of two ways to add this functionality:

• Add a list of sub_transforms to the relations class, which is only applied to submatrices.
• Add an apply_to_sub parameter to the transforms.

I also need to check how this affects the definition of the sub method, especially if it needs to be changed to construct RelationData objects of the subsampled data first, so that the tranforms can safely be applied.

[einbandi]

Maybe the way to go is to have a normalize_rels parameter in the RelationLoss, because that's where the sub method is used.