New tutorial on the transform_output_func parameter of the BatchGenerator class

This tutorial is intended to give a better explanation on how to use the transfom function of the BatchGenerator class. While it is a very powerful function, i feel that documentation for it is poor, specially in figuring out how to use it. In the end, users rely on the default output functions provided by the network as crutches and forget this even exist. Given its importance and power, I think a tutorial with some examples dedicated to how you can use this function is needed.

This tutorial is supposed to accompany the changes I made to ketos in this other merge request for ketos. The changes revert the BatchGenerator class and JointBatchGen to not rely on the internal label mapping. The user is now supposed to do it on his own using the output_transform_func.

What I have added in the tutorial:

1. An overview of the utilities.
2. An example describing the map label process that we did internally before on the BatchGenerator.

I recomend installing the ketos version of the revert_map_labels branch when testing this tutorial as it already has removed the label mapping features.

Any other example you find relevant and wish to add, please go ahead. And change anything you find unclear or formatting.