Extend invariant suite with gradient-checkpointing equivalence

[qgallouedec]

Adds gradient_checkpointing=False as a third member of the existing sft and dpo equivalence classes in tests/invariant/. GC trades compute for memory and should produce trajectories identical to the canonical (gc=False) within fp32 noise; divergence here would be a real bug in the recompute path.

$ pytest tests/invariant/test_invariant.py -m invariant -v -k grad_ckpt
=============================================================== test session starts ===============================================================
platform linux -- Python 3.13.2, pytest-9.0.3, pluggy-1.6.0 -- /fsx/qgallouedec/trl/.venv/bin/python
cachedir: .pytest_cache
rootdir: /fsx/qgallouedec/trl
configfile: pyproject.toml
plugins: cov-7.1.0, rerunfailures-15.1, anyio-4.13.0, datadir-1.8.0, xdist-3.8.0
collected 6 items / 4 deselected / 2 selected                                                                                                     

tests/invariant/test_invariant.py::test_invariant[sft_grad_ckpt] PASSED                                                                     [ 50%]
tests/invariant/test_invariant.py::test_invariant[dpo_grad_ckpt] PASSED                                                                     [100%]

=================================================== 2 passed, 4 deselected in 187.82s (0:03:07) ===================================================

---

Note

Low Risk
Low risk: changes are limited to the invariant test suite, adding extra configurations to validate trajectory equivalence when gradient_checkpointing is disabled, with no production code impact.

Overview
Extends the invariant equivalence tests for both sft and dpo by adding a third member configuration that runs with gradient_checkpointing=False and asserts its loss/grad-norm trajectory matches the class reference within existing tolerances.

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