Add GEM XSum to Full Benchmark

[epavlick]

including COVID+bfp02 challenge sets

[aobaruwa]

I'd be happy to contribute to eval'ing on XSum.

[Shashi456]

The templates for this already exist but nothing specific for the challenge sets mentioned, they can be found here. Can mark this as closed

[jzf2101]

bigscience-workshop/promptsource#745 exists but does does this mean we should close without the PR @awebson ?

[awebson]

Yeah #745 seems good. Ask me again about what's the deal with the challenge set during our next meeting. Closing this now. Thanks!