server : speculative checkpointing (#19493)
* server : speculative decoding using checkpoints
* server : fix draft check with checkpoints
* server : rename spec vars
* server : log levels
* server : refactored spec logic to speculative.cpp
* server : renamed spec checkpoints option
* server : fix spec checkpoints, logging
* speculative : checkpoints with draft model, logging
* server : n_tokens_cur and create_checkpoint in draft
* server : fix server_speculative_callback (slot.id)
* spec : fix ngram-map/begin idx_last_check
* spec : init ckpt (begin() wasn't called)
* chore: update webui build output
* server : restore sampler in spec checkpoint and clear mem
* cont : avoid --spec-use-checkpoints argument
* cont : remove server_prompt_checkpoint_with_size
* spec : rename (leave_draft_state)
* cont : clean-up
* cont : do not ignore partial drafts even if the are short
* cont : spec callback owned by session
* cont : simplify
* cont : avoid empty speculative session
* cont : simplify
* cont : simplify
* cont : enable mtmd speculative decoding
* cont : keep the spec sampler alive
* cont : simplify
* cont : fix nullptr deref + draft checkpoints
* cont : remove common_speculative_accept_response
* cont : remove callback
* cont : simplify
* cont : minor
* cont : simplify
* cont : fix accepted number
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Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>
