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Summary:
Makes lengths input optional for `torchaudio.functional.speed`, `torchaudio.transforms.Speed`, and `torchaudio.transforms.SpeedPerturbation`.

Pull Request resolved: https://github.com/pytorch/audio/pull/3072

Reviewed By: nateanl, mthrok

Differential Revision: D43371406

Pulled By: hwangjeff

fbshipit-source-id: ecb38bcc2bfff5c5a396a37eff238b22238e795a