added Lovasz-loss using pytorch tensors

[gguzzy]

1. Introduced pytorch tensors to Loss, taking advantage of GPUs in torch.
2. Added new example test using pytorch tensors

[gguzzy]

If you want, I can still arrange in a better way the code, aiming at realizing a custom torch loss, would you like to update the code together or can i progress with it? It should still work, but I wanted to be sure on that, what you think? I think it would be really useful for many tasks, that is why I am working on it. @bermanmaxim

Thanks