Add SD task for SUPERB
Note that this subset requires us to:

* generate the LibriMix corpus from LibriSpeech
* prepare the corpus for diarization

As suggested by @lhoestq we should perform these steps locally and add the prepared data to this public repo on the Hub: https://huggingface.co/datasets/superb/superb-data

Then we can use the URLs for the files to load the data in `superb`'s dataset loading script.

For consistency, I suggest we name the folders in `superb-data` in the same way as the configs in the dataset loading script - e.g. use `sd` for speech diarization in both places :) @lewtun @lhoestq: 

I have already generated the LibriMix corpus and prepared the corpus for diarization. The output is 3 dirs (train, dev, test), each one containing 6 files: reco2dur  rttm  segments  spk2utt  utt2spk  wav.scp

Next steps:
- Upload these files to the superb-data repo
- Transcribe the corresponding s3prl processing of these files into our superb loading script

Note that processing of these files is a bit more intricate than usual datasets: https://github.com/s3prl/s3prl/blob/master/s3prl/downstream/diarization/dataset.py#L233


Include the SD (Speaker Diarization) task as described in the [SUPERB paper](https://arxiv.org/abs/2105.01051) and `s3prl` [instructions](https://github.com/s3prl/s3prl/tree/master/s3prl/downstream#sd-speaker-diarization).

Steps:
- [x] Generate the LibriMix corpus
- [x] Prepare the corpus for diarization
- [x] Upload these files to the superb-data repo
- [x] Transcribe the corresponding s3prl processing of these files into our superb loading script
- [ ] README: tags + description sections

Related to #2619.

cc: @lewtun 
