Fix GPU memory leak on StreamReader (#3186)

Summary:
Pull Request resolved: #3186

Fix the GPU memory leak introduced in #3183

The HW frames context is owned by AVCodecContext.
The removed `av_buffer_ref` call increased the ferenrence counting unnecessarily,
and prevented AVCodecContext from feeing the resource.

(Note: this ignores all push blocking failures!)

Reviewed By: nateanl

Differential Revision: D44231876

fbshipit-source-id: 9be2c33049dd02a3fa82a85271de7fb62e5b09ea

Author: mthrok

torchaudio/csrc/ffmpeg/stream_reader/stream_processor.cpp

@@ -157,7 +157,7 @@ AVCodecContextPtr get_codec_ctx(
   configure_codec_context(codec_ctx, params, device);
   open_codec(codec_ctx, decoder_option);
   if (codec_ctx->hw_device_ctx) {
-    codec_ctx->hw_frames_ctx = av_buffer_ref(get_hw_frames_ctx(codec_ctx));
+    codec_ctx->hw_frames_ctx = get_hw_frames_ctx(codec_ctx);
   }
   return codec_ctx;
 }