What this does

This PR makes lerobot/datasets mypy compliant (#1722), by adding or fixing typing in all modules in lerobot/datasets.

How it was tested

Tested typing with uv using:

uv run mypy src/lerobot/datasets/ --config-file=pyproject.toml

Some types were enforced with assert statements. To check that the assertions doesn't break the unit tests, I also tested the repo with uv using:

uv run pytest tests/datasets

How to checkout & try? (for the reviewer)

For typing:

mypy src/lerobot/datasets/ --config-file=pyproject.toml

To check that unit tests are not breaking:

pytest tests/datasets