Fix breadcrumbs for v2.1

[carljparker]

Change the stable version specified in docs/source/conf.py so that breadcrumbs display correctly on TorchAudio documentation pages. I tested on the nightly build using the docs build preview, and tested on the release by building locally on my MBP. See attached screenshots.

[pytorch-bot]

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[mthrok]

@carljparker Thanks for looking into this. I think this need to be documented as release process. To update the 2.1 stable doc do we need to cherry-pick this and rebuild the doc on release branch??

[facebook-github-bot]

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[github-actions]

Hey @None.
You merged this PR, but labels were not properly added. Please add a primary and secondary label (See https://github.com/pytorch/audio/blob/main/.github/process_commit.py).

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Some guidance:

Use 'module: ops' for operations under 'torchaudio/{transforms, functional}', and ML-related components under 'torchaudio/csrc' (e.g. RNN-T loss).

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