Adds More Generative tasks

[hynky1999]

This PR adds more generative tasks and fixes some old ones:

1. COQA -> Is converted into template + we only use the first question unlike previously when all questions were used which doesn't make sense, because any but first question requires context of previous ones
2. DROP -> The metric doesn't really make sense + got converted into tempalte
3. Natural questions -> New task
4. Squad v2 -> New task
5. Jopardy -> New task
6. SImpleQA -> Just added few-shot split

[HuggingFaceDocBuilderDev]

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