mtmd: add batching API (#24384)

* mtmd: add batching API

* wip

* first working version (gemma4v)

* add arg

* nits

* wire up support_batch()

* fix 0.0 output embd

* fix audio

* nits

* refactor a bit

* nits

* fix non-batching case

* fix comment

tools/server/server-http.cpp

@@ -113,7 +113,7 @@ bool server_http_context::init(const common_params & params) {
 #endif
 
     srv->set_default_headers({{"Server", "llama.cpp"}});
-    srv->set_logger(log_server_request);
+    // srv->set_logger(log_server_request); // TODO @ngxson : this is too spamy, no very useful; improve it in the future
     srv->set_exception_handler([](const httplib::Request &, httplib::Response & res, const std::exception_ptr & ep) {
         // this is fail-safe; exceptions should already handled by `ex_wrapper`