image_history = ImageHistory(tensor_board_dir=tb_dir,
        data=v_data, last_step=last_step, draw_interval=100)
model.fit_generator(generator=train_generator,
      steps_per_epoch=steps_per_epoch, epochs=epochs,
      initial_epoch=start_epoch,
      validation_data=validation_data, validation_steps=1,
      callbacks=[tb_cb, image_history])