add a python module called "loss.py" for custom losses published in papers
### Feature request

I'm currently working on a text generation project at work which made me encounter a new loss name "unlikelihood loss".
Thankfully there was an implementation already existed on the internet.
But I was wondering that would be neat to create a separate module just for loss functions specially custom ones published in papers.
I think that would help people out a lot.
I would also like to be the first contribute to this new feature if you think that would be helpful.

[link to the paper](https://arxiv.org/pdf/1908.04319.pdf)

### Motivation

I always run into problem when I  can't understand the mathematics in paper deep enough to be able to implement it myself.
And Hugging Face has already helped a lot.
I think that would be a neat feature to create a separate loss module or script, located in utils or models directory.
That would help people like me that struggle with implementations.

### Your contribution

yes of curse.
I would be glad to contribute to this new feature.

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