Fix some link errors about NNPACK.

[hedaoyuan]

No description provided.

[Xreki]

It is almost LGTM, just with some doubts.

[Xreki]

cpufeatures is a library from ndk, and is special for Android. Just reminding here that we may have a better way to locate it in future.

[Xreki]

In what case there is nnpack_ukernels, and what case there isn't?

[hedaoyuan]

If Android, the nnpack compilation generates the libnnpack_ukernels.a file. If not, there is no such file, all *.o files have been packaged into the libnnpack.a file.

[Xreki]

In this way, we cannot destroy the threadpool_ explicitly, and leave the work to OS?

[hedaoyuan]

Yes, this may not very good. However, the previous version, each NNPACKConvFunction object has a threadpool_ is a bug. When the program running, it will lead to creating many of threads. So, I changed threadpool_ to a static variable.

[Xreki]

Maybe we can define threadpool like this:

template <DeviceType Device>
class NNPACKConvFunction : public ConvFunctionBase {
  ...
private:
  struct ThreadPool {
    ThreadPool() : threadpool_(nullptr) {}
    ~ThreadPool() {
      if (threadpool_) {
        pthreadpool_destroy(threadpool_);
      }
    }
    void create(int num_threads) {
      if (num_threads > 0 && threadpool_ == nullptr) {
        threadpool_ = pthreadpool_create(num_threads);		
        VLOG(3) << "Number of threads "		
                      << pthreadpool_get_threads_count(threadpool_);		
      }
    }

    pthreadpool_t threadpool_;
  };

private:
  static ThreadPool pool_;
};

[hedaoyuan]

Yes, this is better, can you fix it?

[Xreki]

OK.