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
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Co-authored-by: Georgi Gerganov <[email protected]>
