Updated DyNet to PyTorch

[DebadityaPal]

DyNet was quite outdated, so I thought of updating the code to use PyTorch. Merging is not necessary if the change is not desired. But, if anyone wants a PyTorch version, they can find it in this pull request.

[jkkummerfeld]

Thanks for the contribution! I actually have a student who has also been working on a PyTorch version. When I have time, I'll look at both of your versions and come up with a version to go in the repository. I'd like to keep the DyNet one around so that the original code isn't lost (for reproducibility), but having something that works more easily today is definitely appreciated.