Move drain method to private (#2996)

Summary: Pull Request resolved: https://github.com/pytorch/audio/pull/2996

Reviewed By: nateanl

Differential Revision: D42624655

Pulled By: mthrok

fbshipit-source-id: 8273cbfa529fbc2bd28adc9c63ceb9453838baa4

torchaudio/csrc/ffmpeg/stream_reader/stream_reader.h

@@ -106,11 +106,13 @@ class StreamReader {
   int process_packet();
   int process_packet_block(double timeout, double backoff);
 
+ private:
   int drain();
 
   //////////////////////////////////////////////////////////////////////////////
   // Retrieval
   //////////////////////////////////////////////////////////////////////////////
+ public:
   std::vector<c10::optional<torch::Tensor>> pop_chunks();
 };