Added metadata and correct splits for swda.
I will push updates tomorrow. @lhoestq thank you for your comments! I went ahead and fixed the code 😃. Please let me know if I missed anything.
Switchboard Dialog Act Corpus

I made some changes following  @bhavitvyamalik recommendation in #1678:

* Contains all metadata.
* Used official implementation from the [/swda](https://github.com/cgpotts/swda) repo.
* Add official train and test splits used in [Stolcke et al. (2000)](https://web.stanford.edu/~jurafsky/ws97) and validation split used in [Probabilistic-RNN-DA-Classifier](https://github.com/NathanDuran/Probabilistic-RNN-DA-Classifier).