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")
```
4 jobs for inception-resenet-change in 3 minutes and 55 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Build
passed build_linux_37 #232008
docker

00:03:55

35.0%
passed build_linux_38 #232009
docker

00:03:43

36.0%
passed build_macos_intel_37 #232006
macos intel

00:02:00

passed build_macos_intel_38 #232007
macos intel

00:02:00