Llama: fix custom 4D masks, v2 (#30348)
* 4d mask fixes
* Update custom 4D mask logic
* test moved to mixin
* extra tests 4d mask
* upd 4d mask and StaticCache handling
* added Mask4DTestHard to mistral tests
* post-rebase fixes
* test fixes for StaticCache
* make fix-copies
* upd 1 after #30476
* fix common tests
* rm elif attention_mask.dim() == 4:
* tests combined, fixed, mixtral supported
* bigbird style chg reverted
* rm if attention_mask.dim() == 2
* modeling_llama formatting chg
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Co-authored-by: Joao Gante <joao@huggingface.co>
