Clean-up StreamReader/StreamWriter interface (#3328)

Summary:
Pull Request resolved: https://github.com/pytorch/audio/pull/3328

Make the `AVIOContext`-based constructor protected for better encapsulation.
AVFormatContext and optional AVIOContext are managed by StreamReader/Writer, so it's better that they are abstracted away from client code.

Reviewed By: hwangjeff

Differential Revision: D45779629

fbshipit-source-id: 44c31e8af785447cb47aad0c44bf4ecf1aeebeaa

torchaudio/csrc/ffmpeg/stream_writer/stream_writer.h

@@ -26,11 +26,20 @@ class StreamWriter {
   bool is_open = false;
   int current_key = 0;
 
- protected:
   /// @cond
 
+ private:
   explicit StreamWriter(AVFormatContext*);
 
+ protected:
+  /// Construct StreamWriter from custom IO
+  ///
+  /// @param io_ctx Custom IO.
+  /// @param format Specify output format.
+  explicit StreamWriter(
+      AVIOContext* io_ctx,
+      const c10::optional<std::string>& format = c10::nullopt);
+
   /// @endcond
 
  public:
@@ -43,19 +52,6 @@ class StreamWriter {
       const std::string& dst,
       const c10::optional<std::string>& format = c10::nullopt);
 
-  /// @cond
-
-  /// Construct StreamWriter from custom IO
-  ///
-  /// @param io_ctx Custom IO.
-  /// @param format Specify output format.
-  // TODO: Move this into wrapper class.
-  explicit StreamWriter(
-      AVIOContext* io_ctx,
-      const c10::optional<std::string>& format = c10::nullopt);
-
-  /// @endcond
-
   // Non-copyable
   StreamWriter(const StreamWriter&) = delete;
   StreamWriter& operator=(const StreamWriter&) = delete;