Add distributed training support with dask

bob/learn/tensorflow/configs/MirroredStrategy.py

bob/learn/tensorflow/configs/MultiWorkerMirroredStrategy.py

bob/learn/tensorflow/data/generator.py

bob/learn/tensorflow/layers.py

bob/learn/tensorflow/models/__init__.py

bob/learn/tensorflow/models/arcface.py

bob/learn/tensorflow/models/embedding_validation.py

bob/learn/tensorflow/models/iresnet.py

bob/learn/tensorflow/scripts/fit.py

bob/learn/tensorflow/tests/test_arcface.py

bob/learn/tensorflow/utils/keras.py

doc/conf.py

doc/user_guide.rst

examples/mnist_multi_worker_mixed_precision.py

setup.py