Migrate unittest.TestCase to pytest-style test classes

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Several test files still use `unittest.TestCase` from before we adopted pytest. New tests are plain pytest-style classes (see `tests/loss/test_ibot_loss.py` for the pattern: no `TestCase` base, `assert` instead of `self.assertEqual`/`self.assertTrue`, `pytest.mark.parametrize` instead of manual test variants, `pytest.mark.skipif` instead of `unittest.skipUnless`). This migrates the rest.

**Excluded from this issue:** files being deleted by #1939 (API client removal) — `tests/api/test_BitMask.py`, `tests/api/test_rest_parser.py`, `tests/api/test_utils.py`, `tests/api_workflow/mocked_api_workflow_client.py`, `tests/api_workflow/test_api_workflow_client.py`. No point migrating tests that are about to disappear.

Split into one PR per group below — comment to claim a group, don't mix groups in one PR.

- [ ] **Core** — `tests/core/test_Core.py`
- [ ] **Data** — `tests/data/test_LightlyDataset.py`, `tests/data/test_VideoDataset.py`, `tests/data/test_data_collate.py`
- [ ] **Embedding** — `tests/embedding/test_embedding.py`
- [ ] **Models** — `tests/models/test_ModelsBYOL.py`, `tests/models/test_ModelsMoCo.py`, `tests/models/test_ModelsNNCLR.py`, `tests/models/test_ModelsSimCLR.py`, `tests/models/test_ModelsSimSiam.py`, `tests/models/test_ModelUtils.py`, `tests/models/test_ProjectionHeads.py`
- [ ] **Model modules** — `tests/models/modules/test_ijepa_timm.py`, `tests/models/modules/test_masked_autoencoder.py`, `tests/models/modules/test_masked_autoencoder_timm.py`, `tests/models/modules/test_memory_bank.py`
- [ ] **Transforms** — `tests/transforms/test_gaussian_blur.py`, `tests/transforms/test_jigsaw.py`, `tests/transforms/test_Solarize.py`
- [ ] **Utils** — `tests/utils/test_debug.py`, `tests/utils/test_dist.py`, `tests/utils/test_io.py`, `tests/utils/test_version_compare.py`

Mechanical conversion per file:
- `class Foo(unittest.TestCase):` → `class Foo:`
- `setUp`/`tearDown` → pytest fixture
- `self.assertEqual(a, b)` → `assert a == b` (same for `assertTrue`, `assertListEqual`, `assertIsInstance`, etc.)
- `@unittest.skipUnless(cond, msg)` → `@pytest.mark.skipif(not cond, reason=msg)`
- Drop `import unittest` where no longer needed

No behavior change expected — tests should still pass identically after conversion.

Start by choosing one listed group and compare its files with the pytest-style pattern in tests/loss/test_ibot_loss.py. Convert only that group’s unittest.TestCase usage, fixtures, assertions, skips, and imports, then run the group’s tests. Done means the selected tests pass identically without the old unittest patterns.

Written by the indexing model from the issue text.