llama : propagate the results of `graph_compute` (#9525)

* llama: propagating the results of `graph_compute` to the user interface

* llama: reverting kv_cache in case of failed compute

* llama: `llama_kv_cache_state` was removed, only the result of `llama_graph_compute` is returned

* llama: restore a kv_cache in case of failed computation

* llama: correct reverting of the entire batch.
also updates `llama_kv_cache_find_slot`, will correctly count the number of `used` cells for recurrent models

* llama: updated comments

* llama : add comments about KV cache state after error

---------

Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>