Add `examples/multiple-choice/run_swag_no_trainer.py`
This PR adds an example of a multiple-choice task on the SWAG dataset to show the functionalities of the new `accelerate` library.

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**Reviewers:** @sgugger 
Tested on one GPU, two GPUs and TPUs, this runs fine everywhere. So just waiting for the small adjustments and it should be good to be merged :-) <|||||>Thanks a lot!