Training style encoder

[aljox]

Shouldn't we update style encoder when we call g_train_step() with x_ref and not with z_trg?

 if z_trgs is not None:
      f_train_variable = self.mapping_network.trainable_variables
      e_train_variable = self.style_encoder.trainable_variables

      f_gradient = g_tape.gradient(g_loss, f_train_variable)
      e_gradient = g_tape.gradient(g_loss, e_train_variable)

      self.f_optimizer.apply_gradients(zip(f_gradient, f_train_variable))
      self.e_optimizer.apply_gradients(zip(e_gradient, e_train_variable))

Should be:

 if z_trgs is not None:
      f_train_variable = self.mapping_network.trainable_variables
      f_gradient = g_tape.gradient(g_loss, f_train_variable)
      self.f_optimizer.apply_gradients(zip(f_gradient, f_train_variable))
 else:
      e_train_variable = self.style_encoder.trainable_variables
      e_gradient = g_tape.gradient(g_loss, e_train_variable)
      self.e_optimizer.apply_gradients(zip(e_gradient, e_train_variable))