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 [email protected]

• clean-up comments

• fix for multi seq

• apply suggestions && chain-heads comment

• add a reference for chain_heads discussion

Co-authored-by: Aman Gupta [email protected]