Add Multidoc2dial Dataset
This PR adds the MultiDoc2Dial dataset introduced in this [paper](https://arxiv.org/pdf/2109.12595v1.pdf )
@songfeng cc 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
``` Hi @albertvillanova, I have merged master into my PR branch. All tests are passing. 
Please take a look when you get a chance, thanks! 
 Thanks for your feedback @lhoestq. We addressed your comments in the latest commit. Let us know if everything looks okay :) 