Sync dist clarification and consistency

[SkafteNicki]

What does this PR do?

Fixes #20648
Clarify that if the user enables sync_dist=True in self.log then the values are averaged by default (instead of saying it is "reduced"). However, when I checked all the strategies, XLA by default does not average instead it takes the sum. For consistency, I propose that we change the default to be average also for this strategy.

The PR therefore introduce a breaking change in that default averaging for XLA is changed from sum to mean.

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📚 Documentation preview 📚: https://pytorch-lightning--21012.org.readthedocs.build/en/21012/