server : remove n_past (#16818)
* server : remove n_past
* server : replace slot.n_prompt_tokens() with slot.task->n_tokens()
* server : fixes + clean-up
* cont : fix context shift
* server : add server_tokens::pos_next()
Co-authored-by: Xuan-Son Nguyen <son@huggingface.co>
* server : fix pos_next() usage
Co-authored-by: Xuan-Son Nguyen <son@huggingface.co>
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Co-authored-by: Xuan-Son Nguyen <son@huggingface.co>