supporting t5 for question answering
# 🚀 Feature request

Hi, this would be great and appreciated to support t5 model for run_qa.py script, currently this does not support it.

## Motivation

T5 is the state of the art model, for which there are a lot of motivation for people in NLP community to use this model, specially it can handle multiple datasets.


## Your contribution

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Hey @dorooddorood606, 

Since T5 is essentially a text-to-text model, the question-answering task can simply be framed as a seq2seq task. 

I think we could add a `run_qa_seq2seq.py` to https://github.com/huggingface/transformers/tree/master/examples/pytorch/question-answering that is very similar to https://github.com/huggingface/transformers/blob/master/examples/pytorch/summarization/run_summarization.py (we would just need to adapt the dataset to choose from I think). Would you be interested in adding such an example? 

Also pinging @sgugger here to hear his opinion :-)<|||||>We could definitely add this kind of example. We just need a proper dataset, as you mentioned.<|||||>Dear @sgugger @patrickvonplaten 
Thank you very much for considering my request.

In T5 codebase, for superglue-record, they convert each example to multiple ones for each answer choice [1]. During evaluation though they consider all answer choices. I assume this is the case with most of the QA datasets. In T5 script, since we need seq2seq format, I am not sure how I can handle keeping a set of answers.

thank you very much for your comment in advance.

[1] https://github.com/google-research/text-to-text-transfer-transformer/blob/3c58859b8fe72c2dbca6a43bc775aa510ba7e706/t5/data/preprocessors.py#L918<|||||>Cool! @dorooddorood606 would you like to give it a try to make a `run_qa_seq2seq.py` example for SUPERGLUE? Happy to guide you through it :-)<|||||>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.<|||||>unstale, currently wip at #13432 <|||||>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.