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Summary:
In https://github.com/pytorch/audio/issues/2461, `metadata` field was added to StreamInfo.
However, the value attached to this new field was source-level metadata,
while each stream can have different metadata.

* source level metadata
[AVFormatContext->metadata](https://ffmpeg.org/doxygen/4.1/structAVFormatContext.html#a3019a56080ed2e3297ff25bc2ff88adf)
* stream level metadata
[AVFormatContext->streams[]->metadata](https://ffmpeg.org/doxygen/4.1/structAVStream.html#a50d250a128a3da9ce3d135e84213fb82)

This commit moves source level metadata to dedicated method, `get_metadata`, and
fix the stream-level metadata to report stream metadata.

Pull Request resolved: https://github.com/pytorch/audio/pull/2464

Reviewed By: hwangjeff, xiaohui-zhang

Differential Revision: D36995452

Pulled By: mthrok

fbshipit-source-id: 534be1f7feb07790a0ce8624c336cdb7b65a8697