Add SD task for SUPERB

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).

TODO:
- [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
- [x] README: tags + description sections
- ~~Add DER metric~~ (we leave the DER metric for a follow-up PR)

Related to #2619.

Close #2653.

cc: @lewtun 

Comments:
I make a summary about our discussion with @lewtun and @Narsil on the agreed schema for this dataset and the additional steps required to generate the 2D array labels:
- The labels for this dataset are a 2D array:
  Given an example:
  ```python
  {"record_id": record_id, "file": file, "start": start, "end": end, "speakers": [...]}
  ```
  the labels are a 2D array of shape `(num_frames, num_speakers)` where `num_frames = end - start` and `num_speakers = 2`.
- In order to avoid a too large dataset (too large disk space), `datasets` does not store the 2D array label. Instead, we store a compact form:
  ```
  "speakers": [
    {"speaker_id": speaker_0_id, "start": start_0_speaker_0, "end": end_0_speaker_0},
    {"speaker_id": speaker_0_id, "start": start_1_speaker_0, "end": end_1_speaker_0},
    {"speaker_id": speaker_1_id, "start": start_0_speaker_1, "end": end_0_speaker_1},
  ],
  ```
  - Once loaded the dataset, an additional step is required to generate the 2D array label from this compact form
  - This additional step should be a modified version of the s3prl method `_get_labeled_speech`:
    - Original s3prl `_get_labeled_speech` includes 2 functionalities: reading the audio file and transforming it into an array, and generating the label 2D array; I think we should separate these 2 functionalities
    - Original s3prl `_get_labeled_speech` performs 2 steps to generate the labels:
      - Transform start/end seconds (float) into frame numbers (int): I have already done this step to generate the dataset
      - Generate the 2D array label from the frame numbers

I also ping @osanseviero and @lhoestq to include them in the loop.

Here I would like to discuss (and agree) one of the decisions I made, as I'm not completely satisfied with it: to transform the seconds (float) into frame numbers (int) to generate this dataset.

- A priori, the most natural and general choice would be to preserve the seconds (float), because:
  - this is the way the raw data comes from
  - the transformation into frame numbers depends on the sample rate, frame_shift and subsampling

However, I finally decided to transform seconds into frame numbers because:
- for SUPERB, sampling rate, frame_shift and subsampling are fixed (`rate = 16_000`, `frame_shift = 160`, `subsampling = 1`)
- it makes easier the post-processing, as labels are generated from sample numbers:  labels are a 2D array of shape `(num_frames, num_speakers)`
- the number of examples depends on the number of frames:
  - if an example has more than 2_000 frames, then it is split into 2 examples. This is the case for `record_id = "7859-102521-0017_3983-5371-0014"`, which has 2_452 frames and it is split into 2 examples:
    ```
    {"record_id": "7859-102521-0017_3983-5371-0014", "start"= 0, "end": 2_000,...},
    {"record_id": "7859-102521-0017_3983-5371-0014", "start"= 2_000, "end": 2_452,...},
    ```

As I told you, I'm not totally convinced of this decision, and I would really appreciate your opinion.

cc: @lewtun @Narsil @osanseviero @lhoestq 

It makes total sense to prepare the data to be in a format that can actually be used for model training and evaluation. That's one of the roles of this lib :)

So for me it's ok to use frames as a unit instead of seconds. Just pinging @patrickvonplaten in case he has ever played with such audio tasks and has some advice. For the context: the task is to classify which speaker is speaking,  let us know if you are aware of any convenient/standard format for this.

Also I'm not sure why you have to split an example if it's longer that 2,000 frames ?

> Also I'm not sure why you have to split an example if it's longer that 2,000 frames ?

It is a convention in SUPERB benchmark.

Note that if we agree to leave the dataset as it is now, 2 additional custom functions must be used:
- one to generate the 2D array labels
- one to load the audio file into an array, but taking into account start/end to cut the audio

Is there a way we can give these functions ready to be used? Or should we leave this entirely to the end user? This is not trivial...

You could add an example of usage in the dataset card, as it is done for other audio datasets

@albertvillanova this simple function can be edited simply to add the start/stop cuts 

https://github.com/huggingface/transformers/blob/master/src/transformers/pipelines/automatic_speech_recognition.py#L29 

Does this function work on windows ?

Windows ? What is it ? (Not sure not able to test, it's directly calling ffmpeg binary, so depending on the setup it could but can't say for sure without testing)


It's one of the OS we're supposed to support :P (for the better and for the worse)

> Note that if we agree to leave the dataset as it is now, 2 additional custom functions must be used:
> 
> * one to generate the 2D array labels
> * one to load the audio file into an array, but taking into account start/end to cut the audio
> 
> Is there a way we can give these functions ready to be used? Or should we leave this entirely to the end user? This is not trivial...

+1 on providing the necessary functions on the dataset card. aside from that, the current implementation looks great from my perspective!