Add detailed history to CompoundLoss

[einbandi]

Currently it is a bit cumbersome to inspect the components of a CompoundLoss, because each loss's history does not inlcude the weighting. Add a detailed_history function to CompoundLoss, which is just a tensor or dict of each loss component multiplied with its weight.

Once this is implemented, change the description in the Building Blocks document and in the supervised example notebook.