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