Add & test regularizer class hierarchy: L1, L2 & skeleton of MaxNorm
Add support for multiple regularizers in one layer
Simplify the macros in test_regualarizer_as_loss_layer & add more cases
Integrate the Regularizer with the Layer
Skip testing failure cases of test_regularizer_as_loss_layer
Add Regularizer::Regularizer return value to the Backward return value
Rename ret to loss to indicate purpose in Layer::Backward