Implements the original GAN value function as a function to be called in gan_trainer. The function can serve as a template to implement new value functions in RGAN.
GAN_value_fct(real_scores, fake_scores)
The discriminator scores on real examples ($D(x)$)
The discriminator scores on fake examples ($D(G(z))$)
The function returns a named list with the entries d_loss and g_loss