Set filter graph #threads to 1 (#2985)

Summary:
FilterGraph supports multi threading, and by default, the number of threads is determined automatically.

Rather than an automatic behavior, which is unpredictable, it is better to fix the number of threads to 1.

Follow-up: Add an interface to adjust it.

Similar to https://github.com/pytorch/audio/pull/2949.

Pull Request resolved: https://github.com/pytorch/audio/pull/2985

Reviewed By: nateanl

Differential Revision: D42526958

Pulled By: mthrok

fbshipit-source-id: c4f7f95317e93a39378107636a3ca30f6ddfe466

torchaudio/csrc/ffmpeg/filter_graph.cpp

@@ -12,6 +12,8 @@ FilterGraph::FilterGraph(AVMediaType media_type) : media_type(media_type) {
     default:
       TORCH_CHECK(false, "Only audio and video type is supported.");
   }
+
+  pFilterGraph->nb_threads = 1;
 }
 
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