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 <sigbjorn.skjaeret@scala.com>
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Co-authored-by: Sigbjørn Skjæret <sigbjorn.skjaeret@scala.com>
