Use dlopen for FFmpeg

[mthrok]

This commit changes the way FFmpeg extension is built and used.
Instead of linking (LGPL) FFmpeg libraries to torchaudio at build time,
It uses dlopen to search and link them at run time.

For dlopen-ing, we use PyTorch's at::DynamicLibrary class, which provides
portable wrapper.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/audio/3353

• 📄 Preview Python docs built from this PR

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❌ 2 New Failures, 1 Unrelated Failure

As of commit b7f3547:

NEW FAILURES - The following jobs have failed:

• build (3.8) (gh)
• pytorch/audio / conda-py3_8-cpu (gh)

BROKEN TRUNK - The following job failed but were present on the merge base c7ac1af:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• python-source-and-configs / linux-job (gh)

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This pull request was exported from Phabricator. Differential Revision: D46059199

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This pull request was exported from Phabricator. Differential Revision: D46059199

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This pull request was exported from Phabricator. Differential Revision: D46059199

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@mthrok merged this pull request in b14ced1.

[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).

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* 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).

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This pull request has been reverted by ab7a39f.