Merge pop_chunks methods (#3002)

Summary:
Pull Request resolved: https://github.com/pytorch/audio/pull/3002

This commit merges `pop_chunks` and `pop_chunks_with_metadata`.

In #2975 (D42526945 (https://github.com/pytorch/audio/commit/0dd59e0dda22eabf54fc95ad8050094df239bd39)), we updated StreamReader so that it returns PTS.
In that PR, we introduced `pop_chunks_with_metadata` method, so that
the original `pop_chunks` method returns the same type and we could
focus on the PTS logic in the code review.

The commit is landed, now we merge the two methods, so that the original
`pop_chunks` returns Tensor frames and metadata (PTS).

Reviewed By: xiaohui-zhang

Differential Revision: D42662321

fbshipit-source-id: 37ae088bc63fc516ea068698088925e8b31bc0a1

torchaudio/csrc/ffmpeg/stream_reader/stream_reader.cpp

@@ -383,21 +383,9 @@ int StreamReader::drain() {
   return ret;
 }
 
-std::vector<c10::optional<torch::Tensor>> StreamReader::pop_chunks() {
-  std::vector<c10::optional<torch::Tensor>> ret;
-  for (auto& c : pop_chunks_with_metadata()) {
-    if (c) {
-      ret.emplace_back(c->frames);
-    } else {
-      ret.emplace_back();
-    }
-  }
-  return ret;
-}
-
-std::vector<c10::optional<Chunk>> StreamReader::pop_chunks_with_metadata() {
+std::vector<c10::optional<Chunk>> StreamReader::pop_chunks() {
   std::vector<c10::optional<Chunk>> ret;
-  ret.reserve(num_out_streams());
+  ret.reserve(static_cast<size_t>(num_out_streams()));
   for (auto& i : stream_indices) {
     ret.emplace_back(processors[i.first]->pop_chunk(i.second));
   }

torchaudio/csrc/ffmpeg/stream_reader/stream_reader.h

@@ -260,13 +260,9 @@ class StreamReader {
   ///@{
 
   /// Pop one chunk from each output stream if it is available.
-  std::vector<c10::optional<torch::Tensor>> pop_chunks();
+  std::vector<c10::optional<Chunk>> pop_chunks();
 
   ///@}
-
-  /// Pop one chunk from each output stream if it is available.
-  /// TODO: merge this to pop_chunks
-  std::vector<c10::optional<Chunk>> pop_chunks_with_metadata();
 };
 
 } // namespace ffmpeg

torchaudio/csrc/ffmpeg/stream_reader/stream_reader_wrapper.cpp

@@ -106,7 +106,7 @@ int64_t StreamReaderBinding::fill_buffer(
 std::vector<c10::optional<ChunkData>> StreamReaderBinding::pop_chunks() {
   std::vector<c10::optional<ChunkData>> ret;
   ret.reserve(static_cast<size_t>(num_out_streams()));
-  for (auto& c : StreamReader::pop_chunks_with_metadata()) {
+  for (auto& c : StreamReader::pop_chunks()) {
     if (c) {
       ret.emplace_back(std::forward_as_tuple(c->frames, c->pts));
     } else {