Set the default #threads to 1 in StreamWriter (#3370)

Summary:
Similrt to https://github.com/pytorch/audio/issues/2949

Pull Request resolved: https://github.com/pytorch/audio/pull/3370

Differential Revision: D47298746

Pulled By: mthrok

fbshipit-source-id: 0cc0f395772b33f8b2f5f55253d659e451f506c4

torchaudio/csrc/ffmpeg/stream_writer/encode_process.cpp

@@ -184,6 +184,11 @@ void open_codec(
     }
   }
 
+  // Default to single thread execution.
+  if (!av_dict_get(opt, "threads", nullptr, 0)) {
+    av_dict_set(&opt, "threads", "1", 0);
+  }
+
   int ret = avcodec_open2(codec_ctx, codec_ctx->codec, &opt);
   clean_up_dict(opt);
   TORCH_CHECK(ret >= 0, "Failed to open codec: (", av_err2string(ret), ")");

torchaudio/io/_stream_writer.py

@@ -62,7 +62,29 @@ _encoder_option = """Options passed to encoder.
                 To list encoder options for a encoder, you can use
                 ``ffmpeg -h encoder=<ENCODER>`` command.
 
-                Default: ``None``."""
+                Default: ``None``.
+
+                |
+
+                In addition to encoder-specific options, you can also pass options related
+                to multithreading. They are effective only if the encoder support them.
+                If neither of them are provided, StreamReader defaults to single thread.
+
+                ``"threads"``: The number of threads (in str).
+                Providing the value ``"0"`` will let FFmpeg decides based on its heuristics.
+
+                ``"thread_type"``: Which multithreading method to use.
+                The valid values are ``"frame"`` or ``"slice"``.
+                Note that each encoder supports different set of methods.
+                If not provided, a default value is used.
+
+                - ``"frame"``: Encode more than one frame at once.
+                  Each thread handles one frame.
+                  This will increase decoding delay by one frame per thread
+                - ``"slice"``: Encode more than one part of a single frame at once.
+
+                |
+                """
 
 
 _encoder_format = """Format used to encode media.