model: move `load_hparams` and `load_tensors` to per-model definition (#22004)
* git-friendly migration
* add build_graph
* nits
* exclude old code from build
* wip
* add llm_arch_model_i
* prepare downstream functions
* nits
* nits
* wip
* wip
* add back create_tensor_qkv
* fix files missing include
* enforce one llm_build per arch
* cmake: use glob
* missing model params
* nits
* wip
* wip (2)
* wip (3)
* test-llama-archs is happy
* improve switch case
* move more stuff into llm_arch_model_i
* fix downstream code
* nits
* nits (2)
* fix order
* llama_model_base
* LLAMA_LOAD_LOCALS
* small fix
* fix build errors
* auto
* rm migration script and ifdef