Add `OPTForQuestionAnswering`
Author: clementapa
Pull request: yes

# What does this PR do?

This PR adds `OPTForQuestionAnswering` in Transformers. The implementation is based on `BloomForQuestionAnswering` #19310. This introduces a new autoregressive model for question answering tasks in the library.

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Comments:
_The documentation is not available anymore as the PR was closed or merged._

Failures are unrelated to this PR, so merging :-)

Hi @clementapa , 
While adding `OPTForQuestionAnswering` did you test if you were able to train (say, fine-tune on squad) a QA model for any of the opt variants? 

I am getting a fast tokenizer error here: https://github.com/huggingface/transformers/blob/main/examples/pytorch/question-answering/run_qa.py#L345 

Essentially, the `run_qa.py` script requires the model to have a fast tokenizer which is not available for the OPT models.  

Thanks,

Hey! The Fast tokenizer is available for OPT. Make sure you are using main, as a recent issue with automatic conversion for OPT tokenizer was fixed. See #20823