Remove AcceleratorConnector.parallel_devices

[DuYicong515]

What does this PR do?

Fixes #12074 (motivation and alternatives in the issue)
Part of #11449: remove unused properties in accelerator_connector.

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

None

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[DuYicong515]

will hold-off until #12072 merged, as this change depending on that PR, while I don't have access to create a branch and change base branch.

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