Fixed TypeError on 1.7.6 when torch.distributed unavailable

[otaj]

What does this PR do?

#14454 added a subtle bug - when torch.distributed was not available, type of ProcessGroup was set to ..., however it is once used inside Optional (Optional[ProcessGroup]). However, having an Optional of Ellipsis is not allowed by Python type system and results in obscure TypeError.

This was noticed in #14765 (comment)

I do not know how to properly test this. I tried to mock torch.distributed.is_available() and sys.modules and import pytorch_lightning in a test, however this did not yield expected failure on previous version.

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cc @Borda @tchaton @rohitgr7 @akihironitta @awaelchli

[awaelchli]

Thank you @otaj
I also have no idea how to test this