disable use_cache if using gradient checkpointing

[chenzizhao]

What does this PR do?

I disable use_cache when gradient checkpointing is enabled in the new Idefics2 (like in Idefics, Mistral7B, and many others). Previously, the backward pass raises a shape error.

Fixes #30301

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cc: @amyeroberts

[amyeroberts]

Thanks for adding this fix so quickly!