nemotron : support non-standard GQA, BPE tokenizer and domyn-small pre-type (#23343)
* nemotron : support non-standard GQA, BPE tokenizer and domyn-small pre-type
* gguf-py : write Metaspace prepend_scheme as GGUF metadata
* vocab : parse metaspace prepend_scheme into enum, rename key
* vocab : use metaspace prepend_scheme in detokenize, add TODOs for other tokenizers
* vocab : use constant for metaspace escaped space
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Co-authored-by: Martin Cimmino <martin.cimmino@domyn.com>