Fix check_training_gradient_checkpointing

[ydshieh]

What does this PR do?

Currently inour CI runners' environment, there is no pytest-subtests installed. In this case, using subTest with skipTest in it will skip the whole test even if our goal is to just skip that subTest and continue to the next step in the for loop. In check_training_gradient_checkpointing, the first model_class is the base model and it calls skipTest and no other model_class being checked.

However, pytest-subtests has some issues regarding the reporting. A PR is opened pytest-dev/pytest-subtests#169.

In the meantime, let's use continue for now instead of skipTest so we can really test gradient checkpointing.

[HuggingFaceDocBuilderDev]

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[ArthurZucker]

Thanks 🧼