Added support for GPU in ANN and AE

[davidhernandez-cea]

Hi, first of all thank you for your work on these nice packages at mathLab. In this PR is just added GPU support for the ANN and AE classes with the current standard used in PyTorch. We can discuss here if you need more information or if you want me to change/update anything else. Also feel free to close this PR if these changes doesn't fit your desired evolution of your package. However, after implementing these changes I was able to train these NN models with several orders of magnitude faster than with the CPU. I am aware of the existence of the parallel method implemented here, but this just come as a second option to accelerate computations adding GPU capabilities.