Move IJEPAMaskCollator to its own module, keep the old import working (#2020)

* Move IJEPAMaskCollator out of lightly.data.collate

IJEPAMaskCollator is not deprecated but sits in collate.py next to the
collate functions that are, so it would go down with them. Give it its own
module and export it from lightly.data.

Body is unchanged apart from type annotations, which the new module needs
since collate.py is on the mypy exclude list and the new one isn't.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

* Skip IJEPA collator tests on torch<1.11

IJEPAMaskCollator calls torch.utils.data.default_collate, added in torch
1.11, so it does not work on the minimal dependency pin. The class had no
tests before, which is why this never surfaced.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

* Keep IJEPAMaskCollator importable from lightly.data.collate

The class was never deprecated, so moving it out was a breaking change.
Re-export it from the old module behind a FutureWarning pointing at
lightly.data, removed in 1.7.0.

__getattr__ sits in the else branch of a TYPE_CHECKING guard so the module
stays complete for type checkers: the old path resolves to the real class,
and unknown names still error instead of becoming Any.

Also switches the new module to `|` unions and builtin generics.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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Co-authored-by: Claude Opus 5 <noreply@anthropic.com>