text: Title: loss_weights depending on epoch number
  Body: Hi,
  I'm trying to train a multi output nn and I need to change the weight of each loss component depending on the epoch number. In some previous versions of keras I implemented this mechanism by defining each weight of the loss_weights parameter as a K.variable() type, and changing the value with K.set_value() in an on_epoch_begin() method of a custom callback. 
  Now, in keras 3.4 this is not allowed, as loss_weights must be a list/dict of float, so within the callback I can't change the value in place with K.set_value(). Is there something I can do to overcome this issue?
  Thanks