Add `speaker_tasks` datasets folder, add diarization datasets voxconverse/aishell
# What does this PR do ?

Adds a folder to store the `speaker_tasks` datasets. Few things to consider:

* I could create a nested `diarization` folder to be more specific, but I thought just one folder would be fine
* I might've missed some scripts that would be better suited to exist in this new folder

# Changelog 
- Add `speaker_tasks` datasets folder, add diarization datasets voxconverse/aishell.

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**PR Type**:
- [x] New Feature
- [ ] Bugfix
- [ ] Documentation
