Replace SequentialLayer with Sequential Model

The SequentialLayer was introducing more problems than the ones it solved.

Unfortunately, the old checkpoints will not be compatible but they can be easily converted using:

old_model = tf.keras.models.load_model(".../old_checkpoint")
new_model = bob.learn.tensorflow.models.incepetion_resenet_v2.IncepetionResnetV2()
new_model.set_weights(old_model.get_weights)
new_model.save(".../new_checkpoint")