Changes to allow using multiple gpus with default configuration

[g1910]

Hi, I have made changes to the repository to allow using multiple GPUs with Perceptual Similarity implementation. Please review and let me know of any further changes.

[richzhang]

Thank you very much for the pull request. I used it as guidance to add parallelization to the latest commit. I also realized that the PNetLin doesn't need the gpu id information itself and its gpu placement should be controlled from the DistModel class above

[g1910]

Hi Richard, thanks for incorporating the changes. I would have appreciated if you had merged my pull request directly into the code base or suggested further changes for merge so that my contribution could have been officially acknowledged. Since you went ahead and made the changes yourself, it would be nice if you could please acknowledge my contribution somewhere in the repository (perhaps in the ReadMe).

[richzhang]

I got around to cleaning things up and made a pip install. I added you to the readme

[g1910]

Thanks @richzhang! :)