common : n_threads == -1 uses std::thread::hardware_concurrency() (#2347)

* Fix #2345, fix incorrect n_threads

* Update examples/common.cpp

---------

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

examples/common.cpp

@@ -117,6 +117,9 @@ bool gpt_params_parse(int argc, char ** argv, gpt_params & params) {
                 break;
             }
             params.n_threads = std::stoi(argv[i]);
+            if (params.n_threads <= 0) {
+                params.n_threads = std::thread::hardware_concurrency();
+            }
         } else if (arg == "-p" || arg == "--prompt") {
             if (++i >= argc) {
                 invalid_param = true;