Add decoder class (#2042)

Summary:
Part of https://github.com/pytorch/audio/issues/1986. Splitting the PR for easier review.

Add `Decoder` class that manages `AVCodecContext` resource and process input `AVPacket`.
For the overall architecture, see https://github.com/mthrok/audio/blob/ffmpeg/torchaudio/csrc/ffmpeg/README.md.

Note: Without a change to build process, the code added here won't be compiled. The build process will be updated later.
Needs to be imported after https://github.com/pytorch/audio/issues/2041.

Pull Request resolved: https://github.com/pytorch/audio/pull/2042

Reviewed By: carolineechen

Differential Revision: D32933294

Pulled By: mthrok

fbshipit-source-id: e443debadb44d491462fb641cd5b7b20c413b5b9

torchaudio/csrc/ffmpeg/decoder.cpp

+#include <torchaudio/csrc/ffmpeg/decoder.h>
+
+namespace torchaudio {
+namespace ffmpeg {
+
+////////////////////////////////////////////////////////////////////////////////
+// Decoder
+////////////////////////////////////////////////////////////////////////////////
+Decoder::Decoder(AVCodecParameters* pParam) : pCodecContext(pParam) {}
+
+int Decoder::process_packet(AVPacket* pPacket) {
+  return avcodec_send_packet(pCodecContext, pPacket);
+}
+
+int Decoder::get_frame(AVFrame* pFrame) {
+  return avcodec_receive_frame(pCodecContext, pFrame);
+}
+
+} // namespace ffmpeg
+} // namespace torchaudio

torchaudio/csrc/ffmpeg/decoder.h

+#pragma once
+
+#include <torchaudio/csrc/ffmpeg/ffmpeg.h>
+
+namespace torchaudio {
+namespace ffmpeg {
+
+class Decoder {
+  AVCodecContextPtr pCodecContext;
+
+ public:
+  // Default constructable
+  Decoder(AVCodecParameters* pParam);
+  // Custom destructor to clean up the resources
+  ~Decoder() = default;
+  // Non-copyable
+  Decoder(const Decoder&) = delete;
+  Decoder& operator=(const Decoder&) = delete;
+  // Movable
+  Decoder(Decoder&&) = default;
+  Decoder& operator=(Decoder&&) = default;
+
+  // Process incoming packet
+  int process_packet(AVPacket* pPacket);
+  // Fetch a decoded frame
+  int get_frame(AVFrame* pFrame);
+};
+
+} // namespace ffmpeg
+} // namespace torchaudio