Fix SaveConfigCallback with ddp spawn

[carmocca]

What does this PR do?

A CLI test using different strategies (ddp spawn, ddp) was silently broken accidentally in this PR:
https://github.com/PyTorchLightning/pytorch-lightning/pull/9931/files#diff-99e49066f136ed08cfc2c4ed81ff44eb71414ab3e600a62d278e4bd613a67997R584-R585
due to a bug in the AcceleratorConnector

Since the test was not working as expected, #10896 broke the SaveConfigCallback without noticing because now setup is called AFTER processes are spawned.

Then, #11448 fixed the AcceleratorConnector bug which uncovered the failing CLI test.

This PR fixes the test and uses an internal hook (aka a hack) to support DDP spawn again.
Note that the bug is unreleased.

A longer-term fix will be to make ArgumentParser pickleable or to have the parser expose a static save function. @mauvilsa

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

None.

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cc @carmocca @mauvilsa @justusschock @kaushikb11 @awaelchli @akihironitta @rohitgr7