on_train_end callback is not called if training is stopped early

[neggert]

            # early stopping
            met_min_epochs = epoch >= self.min_epochs - 1
            if self.enable_early_stop and (met_min_epochs or self.fast_dev_run):
                should_stop = self.early_stop_callback.on_epoch_end(epoch=epoch,
                                                                    logs=self.callback_metrics)
                # stop training
                stop = should_stop and met_min_epochs
                if stop:
                    self.main_progress_bar.close()
                    return

        self.main_progress_bar.close()

        model.on_train_end()

[williamFalcon]

@neggert good catch. PR?