Reduce memory use of KTH data loader by factor 4

[lkoelman]

Hi there, this edit reduces the memory footprint of the KTH data loader by a factor of four. The original version did not run on systems with <= 16 GB, whereas this version does. Images are stored as grayscale int8 until they are loaded into a batch (since they are grayscale anyway).

This also makes the code suitable to be run on Google Colab (free version).