hparams : refactor hparams.n_layer (#24060)

• hparams : refactor hparams.n_layer

• cont : remove n_layer_kv(), use n_layer_all instead

• cont : type consistency

• pi : update SYSTEM.md

• models : fix Step3.5 MTP

• cont : remove duplicate switch cases

• cont : explicitly set false to extra layers for is_swa and is_recr

• cont : fix nextn layer count handling

Co-authored-by: Sigbjørn Skjæret [email protected]

Co-authored-by: Sigbjørn Skjæret [email protected]