llama : support models without vocabulary (#5798)

* additional methods to read model and ctx parameters

* vocab size as a part of a model metadata

* models without vocabulary, convert.py part

* models without vocabulary, llama.cpp part

* PR clean up

* converter scrypt fixes

* llama_vocab_type update (renamed the new key)

* pr review fixes

* revert function renaming

* one more NoVocab assert