Build and use GPU-enabled FFmpeg in doc CI

[mthrok]

This commit add the step to build FFmpeg with GPU decoder in build_doc job so that we can use GPU decoder/encoder in documentations.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/audio/3045

• 📄 Preview Python docs built from this PR

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❌ 1 New Failure

As of commit bce5345:

NEW FAILURE - The following job has failed:

• build (3.8) (gh)

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[facebook-github-bot]

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[facebook-github-bot]

@mthrok merged this pull request in 0db5ab2.

[github-actions]

Hey @mthrok.
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).

Things in "examples" directory:

• 'recipe' is applicable to training recipes under the 'examples' folder,
• 'tutorial' is applicable to tutorials under the “examples/tutorials” folder
• 'example' is applicable to everything else (e.g. C++ examples)
• 'module: docs' is applicable to code documentations (not to tutorials).
  Regarding examples in code documentations, please also use 'module: docs'.

Please use 'other' tag only when you’re sure the changes are not much relevant to users, or when all other tags are not applicable. Try not to use it often, in order to minimize efforts required when we prepare release notes.

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When preparing release notes, please make sure 'documentation' and 'tutorials' occur as the last sub-categories under each primary category like 'new feature', 'improvements' or 'prototype'.

Things related to build are by default excluded from the release note, except when it impacts users. For example:
* Drop support of Python 3.7.
* Add support of Python 3.X.
* Change the way a third party library is bound (so that user needs to install it separately).