Add namespace when loading wikitext dataset

[echarlaix]

fix https://github.com/huggingface/optimum/actions/runs/26833981869/job/79122752650

FAILED tests/gptq/test_quantization.py::GPTQDataTest::test_dataset_0_wikitext2 - huggingface_hub.errors.HfUriError: Invalid HF URI 'hf://datasets/wikitext@b08601e04326c79dfdd32d625aee71d232d685c3/.huggingface.yaml'. Repository id must be 'namespace/name', got 'wikitext'.

[HuggingFaceDocBuilderDev]

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[IlyasMoutawwakil]

LGTM ! thanks for the fix