Fix CI slow test ValueError: Unknown loss type: dapo

[albertvillanova]

Fix CI slow test: https://github.com/huggingface/trl/actions/runs/18399649932/job/52425772805

ValueError: Unknown loss type: dapo

FAILED tests/slow/test_grpo_slow.py::TestGRPOTrainerSlow::test_training_with_liger_grpo_loss_and_peft_0_trl_internal_testing_tiny_LlamaForCausalLM_3_2 - ValueError: Unknown loss type: dapo
FAILED tests/slow/test_grpo_slow.py::TestGRPOTrainerSlow::test_training_with_liger_grpo_loss_and_peft_1_trl_internal_testing_tiny_MistralForCausalLM_0_2 - ValueError: Unknown loss type: dapo

Follow-up to:

• Fix CI ValueError: Unknown loss type: dapo #4173

[albertvillanova]

Test passed: https://github.com/huggingface/trl/actions/runs/18407670353/job/52451408042?pr=4254

pytest -m "slow" tests/slow/test_grpo_slow.py::TestGRPOTrainerSlow::test_training_with_liger_grpo_loss_and_peft_0_trl_internal_testing_tiny_LlamaForCausalLM_3_2 --report-log "slow_tests.log"
============================= test session starts ==============================
platform linux -- Python 3.11.13, pytest-8.4.2, pluggy-1.6.0
rootdir: /__w/trl/trl
configfile: pyproject.toml
plugins: anyio-4.11.0, cov-7.0.0, rerunfailures-15.1, xdist-3.8.0, reportlog-0.4.0
collected 1 item

tests/slow/test_grpo_slow.py .                                           [100%]

[HuggingFaceDocBuilderDev]

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