Add TasksetSampler (compatible with non-classification tasks)

[seba-1511]

Description

Adds a sampler over tasks compatible with PyTorch's DataLoader (incl. num_workers > 1) which doesn't assume classification tasks.

If used for classification tasks (or others), it does not support any DataDescription transform (e.g., remapping the labels to 0-4 for a 5w5s tasks, or sorting the samples by labels).

This PR also deprecates the name TaskDataset in favor of Taskset, and includes some warning utilities. (TODO: decide if those changes are wise.)

Contribution Checklist

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• [ ] My contribution is listed in CHANGELOG.md with attribution.
• [x] My contribution modifies code in the main library.
• [ ] My modifications are tested.
• [ ] My modifications are documented.

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