GPT-model attention heads pruning example
# What does this PR do?

This script is adapted from the [BERT attention heads pruning code](https://github.com/huggingface/transformers/blob/master/examples/research_projects/bertology/run_bertology.py) AKA Bertology to make it possible to prune GPT-model heads as well.

It basically works the same way as run_bertology.py, but can deal with GPT-models.

One last detail: could you run `make style` on your branch? There seems to be some bad formatting.<|||||>> One last detail: could you run `make style` on your branch? There seems to be some bad formatting.

I did, but let me recheck it