Skip test_contrastive_generate for TFXLNet

[ydshieh]

What does this PR do?

This model has special cache mechanism and test_contrastive_generate fails for more than 8 month. When I remove the argument penalty_alpha from the kwargs, it is able to generate something.

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[ydshieh]

For the record, it starts to fail on commit a6c850e4 (#20994) but with a different error TypeError: prepare_inputs_for_generation() got multiple values for keyword argument 'use_cache'. Then in #21149, we get the current error.

[gante]

LGTM, thanks for fixing 👍