Add distributed training support with dask

Closes #87

See merge request !97

Replace SequentialLayer with Sequential Model

See merge request !95

Updated ARCFACE

See merge request !96

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")
```

Properly implemented resnet50 and resnet101

See merge request !94

ArcFace/SphereFace Loss

See merge request !93

Apply suggestion to bob/learn/tensorflow/layers.py

Apply suggestion to bob/learn/tensorflow/layers.py

Rewiring Arcface to make it simpler

Rewiring Arcface to make it simpler

Few changes

Removing test

Removing test

Added test case

Add necessary files to train DeepPixBis

See merge request !92

Fixed issues with TF1 compatibility

See merge request !91

Cleanup

See merge request !89