[RUNTIME] Fast path for single thread run to allow app level threading (

apache#7454)

* Fast path for single thread run to allow app level threading

* add sync counter to avoid error in one of tests

src/runtime/thread_pool.cc

@@ -363,21 +363,30 @@ TVM_REGISTER_GLOBAL("runtime.config_threadpool").set_body([](TVMArgs args, TVMRe
 }  // namespace tvm
 
 int TVMBackendParallelLaunch(FTVMParallelLambda flambda, void* cdata, int num_task) {
+  int num_workers = tvm::runtime::threading::MaxConcurrency();
+  if (num_workers == 1) {
+    std::atomic<int32_t> sync_counter{0};
+    TVMParallelGroupEnv env;
+    env.num_task = 1;
+    env.sync_handle = &sync_counter;
+    (*flambda)(0, &env, cdata);
+    return 0;
+  } else {
 #if !TVM_THREADPOOL_USE_OPENMP
-  int res = tvm::runtime::ThreadPool::ThreadLocal()->Launch(flambda, cdata, num_task, 1);
-  return res;
+    int res = tvm::runtime::ThreadPool::ThreadLocal()->Launch(flambda, cdata, num_task, 1);
+    return res;
 #else
-  int num_workers = tvm::runtime::threading::MaxConcurrency();
-  if (num_task == 0) num_task = num_workers;
-  omp_set_num_threads(num_task);
+    if (num_task == 0) num_task = num_workers;
+    omp_set_num_threads(num_task);
 #pragma omp parallel num_threads(num_task)
-  {
-    TVMParallelGroupEnv env;
-    env.num_task = num_task;
-    (*flambda)(omp_get_thread_num(), &env, cdata);
-  }
-  return 0;
+    {
+      TVMParallelGroupEnv env;
+      env.num_task = num_task;
+      (*flambda)(omp_get_thread_num(), &env, cdata);
+    }
+    return 0;
 #endif
+  }
 }
 
 int TVMBackendParallelBarrier(int task_id, TVMParallelGroupEnv* penv) {