spec : Support Step3.5/3.7 flash mtp3 (#24340)
* add mtp_layer_offset + include nextn flags in graph reuse
* add llama_set_mtp_layer_offset + llama_model_n_nextn_layer API
* offset head select + require all MTP blocks
* speculative multi-head process()
* speculative multi-head draft()
* gather outputs via inp_out_ids
* cleanup
* fix core
* minor cleanup
* merged draft_multi_head into draft()
* mtp rename nextn
* Apply suggestions from code review
Co-authored-by: Aman Gupta <amangupta052@gmail.com>
* clean-up comments
* fix for multi seq
* apply suggestions && chain-heads comment
* add a reference for chain_heads discussion
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Co-authored-by: Aman Gupta <amangupta052@gmail.com>
