Make pytorch an optional dependency in klay_beam

[CharlesHolbrow]

Closes #31

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My plan is to keep LoadWithTorchaudio in the klay_beam package, so that any jobs that benefit from audio in torch.Tensors don't have to re-implement it. This will also make it easier to migrate existing jobs to klay_beam 1.0, because they won't have to all switch to LoadWithLibrosa simultaneously.

There will be breaking name changes to other methods that depend on torchaudio. klay_beam methods that depends on torchaudio should indicate this with thier function name (ex ResampleWithTorchaudio)

[CharlesHolbrow]

Thanks a lot, Max! There are few small thing that you commented on that are coming in future PRs.