Add Multidoc2dial Dataset

[sivasankalpp]

This PR adds the MultiDoc2Dial dataset introduced in this paper

[sivasankalpp]

@songfeng cc

[albertvillanova]

Hi @sivasankalpp, thanks for your PR.

There was a bug in TensorFlow/Keras. We have made a temporary fix in our master branch. Please, merge master into your PR branch, so that the CI tests pass.

git checkout multidoc2dial
git fetch upstream master
git merge upstream/master

[sivasankalpp]

Hi @albertvillanova, I have merged master into my PR branch. All tests are passing.
Please take a look when you get a chance, thanks!

[lhoestq]

This is awesome ! Thanks for adding this dataset. And good job with the dataset card and the dataset script :)

I just added some minor comments here and there:

[sivasankalpp]

Thanks for your feedback @lhoestq. We addressed your comments in the latest commit. Let us know if everything looks okay :)

[lhoestq]

Cool thanks a lot ! LGTM :)