Cross validation

This MR adds the following

• Adds cross-validation using a validation set; the best model based on the lowest validation loss is saved automatically now
• Adds trainer, extractor, training script and unit tests for FASNet architecture
• Small change in MCCNN architecture; added a flag for selecting whether to use sigmoid in eval phase.

bob/learn/pytorch/architectures/FASNet.py

@@ -8,7 +8,7 @@ class FASNet(nn.Module):
 	"""PyTorch Reimplementation of Lucena, Oeslle, et al. "Transfer learning using 
 	convolutional neural networks for face anti-spoofing."
 	International Conference Image Analysis and Recognition. Springer, Cham, 2017.
-	eferenced from keras implementation: https://github.com/OeslleLucena/FASNet
+	Referenced from keras implementation: https://github.com/OeslleLucena/FASNet
 
 	Attributes:
 		pretrained: bool