server : implement universal assisted decoding (#12635)

server : implement universal assisted decoding (#12635)

* llama-server : implement universal assisted decoding

* Erase prompt tail for kv-cache

* set vocab_dft_compatible in common_speculative

* rename ctx_main to ctx_tgt

* move vocab_dft_compatible to spec struct

* clear mem_dft, remove mem

* detokenize id_last for incompatible models

* update comment

* add --spec-replace flag

* accept special tokens when translating between draft/main models

* Escape spec-replace

* clamp draft result to size to params.n_draft

* fix comment

* clean up code

* restore old example

* log common_speculative_are_compatible in speculative example

* fix

* Update common/speculative.cpp

Co-authored-by: Georgi Gerganov <redacted>
* Update common/speculative.cpp

Co-authored-by: Georgi Gerganov <redacted>
* Update common/speculative.cpp

Co-authored-by: Georgi Gerganov <redacted>
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Co-authored-by: Georgi Gerganov <redacted>