Cleanup: DataLoader in Trainer

[samuelburbulla]

Cleanup: DataLoader in Trainer

Description

As everything related to training moved into the Trainer class, it is consequent to also move the DataLoader to the 'fit' methods as it, particularly, can take care of parallel training if available.

Which issue does this PR tackle?

• DataLoader is instantiated everywhere but could be part of the 'Trainer' class.

How does it solve the problem?

• Move DataLoader to the 'fit' method of the Trainer.

How are the changes tested?

• DataLoader removed from (almost) all tests.

Checklist for Contributors

• [x] Scope: This PR tackles exactly one problem.
• [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.