Add query methods to FilterGraph (#2976)

Summary:
This commit add methods to query output configuration from FilterGraph object.
* time_base -> required to compute PTS of output frame
* sample_rate, num_channels -> required to compute PTS and pre allocate buffers for audio.

Pull Request resolved: https://github.com/pytorch/audio/pull/2976

Reviewed By: xiaohui-zhang

Differential Revision: D42466744

Pulled By: mthrok

fbshipit-source-id: dd27109819bfb1fbe37b8233dd6a5e4224fe3f6c

torchaudio/csrc/ffmpeg/filter_graph.cpp

@@ -167,6 +167,25 @@ void FilterGraph::create_filter() {
   // av_free(static_cast<void*>(desc));
 }
 
+//////////////////////////////////////////////////////////////////////////////
+// Query methods
+//////////////////////////////////////////////////////////////////////////////
+AVRational FilterGraph::get_output_timebase() const {
+  TORCH_INTERNAL_ASSERT(buffersink_ctx, "FilterGraph is not initialized.");
+  return buffersink_ctx->inputs[0]->time_base;
+}
+
+int FilterGraph::get_output_sample_rate() const {
+  TORCH_INTERNAL_ASSERT(buffersink_ctx, "FilterGraph is not initialized.");
+  return buffersink_ctx->inputs[0]->sample_rate;
+}
+
+int FilterGraph::get_output_channels() const {
+  TORCH_INTERNAL_ASSERT(buffersink_ctx, "FilterGraph is not initialized.");
+  return av_get_channel_layout_nb_channels(
+      buffersink_ctx->inputs[0]->channel_layout);
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 // Streaming process
 //////////////////////////////////////////////////////////////////////////////

torchaudio/csrc/ffmpeg/filter_graph.h

@@ -49,6 +49,13 @@ class FilterGraph {
 
   void create_filter();
 
+  //////////////////////////////////////////////////////////////////////////////
+  // Query methods
+  //////////////////////////////////////////////////////////////////////////////
+  [[nodiscard]] AVRational get_output_timebase() const;
+  [[nodiscard]] int get_output_sample_rate() const;
+  [[nodiscard]] int get_output_channels() const;
+
   //////////////////////////////////////////////////////////////////////////////
   // Streaming process
   //////////////////////////////////////////////////////////////////////////////