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

@@ -201,7 +201,7 @@ class ModifiedLogitsHead(tf.keras.layers.Layer):
 
 class AngularHead(tf.keras.layers.Layer):
     """
-    Implements :math:`\frac{x_i}{||x_i||}\frac{W}{||W||}`.
+    Implements :math:`\\frac{x_i}{||x_i||}\\frac{W}{||W||}`.
     This is the base operation for equations (3) and (4) of the paper 
     `ArcFace: Additive Angular Margin Loss for Deep Face Recognition <https://arxiv.org/abs/1801.07698>`_