Add Multiple Choice Pytorch Example for Hellaswag
Author: prajjwal1
Pull request: yes

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Fixes # (issue)
Added example for `hellaswag` dataset. 

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Models:

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Comments:
One thing that I was wondering is to replacing `swag` script with `hellaswag` mainly because of the reason that `swag` has been shown to contain artifacts in the endings itself. This was shown in `hellaswag` paper itself, so example of `hellaswag` would be more useful to researchers compared to `swag` example. `swag` is comparatively less used in papers nowadays. If you don't feel this way, feel free to close this PR.

I'm fine with switching datasets, if you want to amend your PR in that sense :-)

I've replaced `swag` with `hellaswag` to switch these datasets.

This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.