optimize thread pool code

test=develop

paddle/fluid/framework/threadpool.cc

@@ -70,8 +70,10 @@ ThreadPool::~ThreadPool() {
 
 void ThreadPool::TaskLoop() {
   while (true) {
-    std::unique_lock<std::mutex> lock(mutex_);
+    Task task;
 
+    {
+      std::unique_lock<std::mutex> lock(mutex_);
       scheduled_.wait(
           lock, [this] { return !this->tasks_.empty() || !this->running_; });
 
@@ -84,9 +86,9 @@ void ThreadPool::TaskLoop() {
       }
 
       // pop a task from the task queue
-    auto task = std::move(tasks_.front());
+      task = std::move(tasks_.front());
       tasks_.pop();
-    lock.unlock();
+    }
 
     // run the task
     task();