New place to put extactors

Created a place to hold the code for extractors based on architectures defined in bob.learn.pytorch.architectures.

@ageorge @onikisins @ssarfjoo : If you implement a new extractor, it should be placed here (don't worry about what has already been done in bob.ip.pytorch_extractor, I'll take care of that when I'll have some more time)

Thanks

bob/learn/pytorch/extractor/image/__init__.py

+
+__all__ = [_ for _ in dir() if not _.startswith('_')]
+