metal : unify mul_mv_id kernels (#6556)

tests/test-backend-ops.cpp

@@ -2014,6 +2014,7 @@ static bool test_backend(ggml_backend_t backend, test_mode mode, const char * op
             for (int n_mats : {2, 4, 8}) {
                 for (int id = 0; id < n_mats; id++) {
                     for (bool v : {false, true}) {
+                        test_cases.emplace_back(new test_mul_mat_id(type_a, type_b, n_mats, id, 16,  1, 256, v));
                         test_cases.emplace_back(new test_mul_mat_id(type_a, type_b, n_mats, id, 16, 16, 256, v));
                     }
                 }