llava : introduce libmtmd (#12849)

* wip llava2

* migrated gemma3 to llava2

* add timings

* correct pre/postfix

* fix missing include

* fix compilation unused var warn

* update llava2_tokenize

* change name llava2 --> mtmd

* improve api

* refine helpers

* Update examples/llava/mtmd.cpp

Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>

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Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>