llama: add llama_batch_ext (#24669)
* (wip) add llama_batch_ext
* wip
* updated design
* updated impl
* change signature
* unused var
* demo common_prompt_batch_decode
* fix pos
* tmp disable test-batch-alloc
* fix compat
* nits: add const
* no more pos_max
* add comment about llama_batch_ext_set_embd_state
* handle n_embd_out properly
* rename api --> embd_token
* llama_embd
* stub llama_batch_ext_set_embd_state
* support both token + embd + state in batch
* llama_batch_ext_add_embd
* upstream some changes
* nits
* fix test-batch-alloc
* add test for compat
