CUDA enable

[EnricoBeltramo]

I compiled this project on Windows 10, with python 3.5 and tensorflow gpu 1.12.0.
Tensorflow work correctly with CUDA and GPU on my PC, but starting a train on this project seem the CUDA and GPU are disabled. There is a reason? Is it possible to enable it?

[xbpeng]

Our networks are fairly small, so they don't benefit as much from GPU accelerations, so for the most part we just train on the CPU. Also tensoflow-gpu doesn't play well with multiprocessing using MPI, and need some additional overhead to have multiple processes share a GPU.