ArcFace/SphereFace Loss

Implemented 2 losses and 1 layer from the arcface paper: https://arxiv.org/abs/1801.07698

Implemented 1 loss and 1 layer from the sphere face paper: https://arxiv.org/pdf/1704.08063.pdf

Implemented a keras model that does the validation in the test step

bob/learn/tensorflow/layers.py

@@ -166,7 +166,7 @@ class ModifiedLogitsHead(tf.keras.layers.Layer):
     """
     Implements the modified logit from equation (5) of `SphereFace: Deep Hypersphere Embedding for Face Recognition <https://arxiv.org/pdf/1704.08063.pdf>`_
 
-    It basically transforms the regular logit function to :math:`||x_i||cos(\theta_{yi})`, where :math:`\theta_{yi}=||x_i||_2^2||W||_2^2`
+    It basically transforms the regular logit function to :math:`||x_i||cos(\\theta_{yi})`, where :math:`\\theta_{yi}=||x_i||_2^2||W||_2^2`
 
     Parameters
     ----------