Add example code for CRF heads 


# Add example `crf/` with heads that force structural output dependencies. 

(Mostly a note to myself as a side project)

## Model description

As requested in: https://github.com/huggingface/transformers/pull/3009 there are some tasks and languages where it is useful to have final layer structural dependencies. 

Using https://github.com/harvardnlp/pytorch-struct/ we can add these with minimal changes to the code and no new model parameters. 

Target:
* example code for ner / parsing (sota). 


Also https://github.com/huggingface/transformers/pull/2249/<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
