mtmd : support Qwen 2.5 Omni (input audio+vision, no audio output) (#13784)

mtmd : support Qwen 2.5 Omni (input audio+vision, no audio output) (#13784)

* mtmd : allow multiple modalities at the same time

* refactor mtmd tokenizer

* fix compile

* ok, missing SinusoidsPositionEmbedding

* first working version

* fix style

* more strict validate of n_embd

* refactor if..else to switch

* fix regression

* add test for 3B

* update docs

* fix tokenizing with add_special

* add more tests

* fix test case "huge"

* rm redundant code

* set_position_mrope_1d rm n_tokens