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Summary:
This commit adds the following behavior to `seek` so that `seek`
works after a frame is decoded.

1. Flush the decoder buffer.
2. Recreate filter graphs (so that internal state is re-initialized)
3. Discard the buffered tensor. (decoded chunks)

Also it disallows negative values for seek timestamp.

Pull Request resolved: https://github.com/pytorch/audio/pull/2264

Reviewed By: carolineechen

Differential Revision: D34497826

Pulled By: mthrok

fbshipit-source-id: 8b9a5bf160dfeb15f5cced3eed2288c33e2eb35d