Rename special to standalone

[carmocca]

What does this PR do?

Special was a confusing name. This PR uses standalone which represents better what it does.

Other candidates were:

single_process=True
split_process=True
separate_process=True

Does your PR introduce any breaking changes? If yes, please list them.

None

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• [n/a] Did you write any new necessary tests? (not for typos and docs)
• [n/a] Did you verify new and existing tests pass locally with your changes?
• Did you list all the breaking changes introduced by this pull request?
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PR review

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cc @carmocca @akihironitta @Borda @justusschock @awaelchli

[github-actions]

Build Error! No Linked Issue found. Please link an issue or mention it in the body using #<issue_id>

[rohitgr7]

small comments!