Skip private model loading for external contributors

[echarlaix]

https://github.com/huggingface/optimum/actions/runs/12406440330/job/34639615964?pr=2114

FAILED tests/onnxruntime/test_modeling.py::ORTModelIntegrationTest::test_load_model_from_hub_private - huggingface_hub.errors.RepositoryNotFoundError: 401 Client Error. (Request ID: Root=1-6763cff5-38ad3a1d75a3598d6b742f30;13c34049-a28a-43d1-a22b-e79829428ccf)

Repository Not Found for url: https://huggingface.co/api/models/optimum-internal-testing/tiny-random-phi-private/tree/onnx?recursive=True&expand=False.
Please make sure you specified the correct `repo_id` and `repo_type`.
If you are trying to access a private or gated repo, make sure you are authenticated.
Invalid username or password.

[IlyasMoutawwakil]

There's a require_hf_token decorator in https://github.com/huggingface/optimum/blob/main/optimum/utils/testing_utils.py#L88
it's used in some places but it checks for HF_AUTH_TOKEN instead of HF_HUB_READ_TOKEN 🤔 should we use that decorator instead ?

[echarlaix]

Here the issue seems that the token is set as an empty string instead of being set to None (so the same issue will happen for require_hf_token). Let's see if this fixes it, in which case I'll update require_hf_token