mtmd: add --mmproj-device argument (#23255)

* feat: add --mmproj-device arg & backwards compatible MTMD_BACKEND_DEVICE env var

* feat: load mmproj device backend immediately, add -mmdev shortflag

* fix: its a pointer now get the name

* clean up

* gen docs

* nits

---------

Co-authored-by: Xuan Son Nguyen <son@huggingface.co>

tools/tts/tts.cpp

@@ -86,6 +86,7 @@ int main(int argc, char ** argv) {
 
     mtmd_context_params mtmd_params = mtmd_context_params_default();
     mtmd_params.use_gpu = params.mmproj_use_gpu;
+    mtmd_params.device  = params.mmproj_device;
     mtmd::context_ptr mctx(mtmd_init_from_file(params.mmproj.path.c_str(), model, mtmd_params));
     if (!mctx) {
         LOG_ERR("failed to load mmproj %s\n", params.mmproj.path.c_str());