[generate] shape checks in tests compatible with fixed-length caches (+ some minor fixes) (#35993)
* shape checks compatible with static cache
* add test
* tmp
* manually turn on eager attn when we want to output attn
* typo
* generalize to encoder-decoder models
* force compilation on cpu
* tmp commit
* fix static cache shape checks
* models with odd caches
* fix copies
* shorter cache search loop
* use decoder_past_key_values everywhere
* better test variable names and comments
* signature
* rename _check_outputs into _check_generate_outputs
* add comments
* HybridCache future test note