Cleanup: Use pytorch dataset and data loader

[samuelburbulla]

Description

Which issue does this PR tackle?

• Custom DataSet is unnecessary and circumvents the use of pyTorch dataset and data loader implementations.
• Jupyter Notebooks are not tested.

How does it solve the problem?

• Removes DataSet class.
• Replaces DataSet by pyTorch Dataset and DataLoader in all examples and tests.
• Adds notebook tests to CI (because they also should use DataLoader now).
• Removes automatic device management (as we let DataLoader manage this now)
• Removes automatic batching in operator forward (as we assume DataLoader input now)

How are the changes tested?

• Checked all examples and tests.
• Added CI testing of notebooks.

Checklist for Contributors

• [x] Scope: This PR tackles exactly one problem. (yes, but many things are directly related)
• [x] Conventions: The branch follows the feature/title-slug convention.
• [x] Conventions: The PR title follows the Bugfix: Title convention.
• [x] Coding style: The code passes all pre-commit hooks.
• [x] Documentation: All changes are well-documented.
• [x] Tests: New features are tested and all tests pass successfully.
• [x] Changelog: Updated CHANGELOG.md for new features or breaking changes.
• [x] Review: A suitable reviewer has been assigned.

Checklist for Reviewers:

• [x] The PR solves the issue it claims to solve and only this one.
• [x] Changes are tested sufficiently and all tests pass.
• [x] Documentation is complete and well-written.
• [x] Changelog has been updated, if necessary.