Sync dist clarification and consistency (#21012)

* change default reduction in xla to mean

* clarify docs that sync_dist applies average

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Bhimraj Yadav <bhimrajyadav977@gmail.com>
Co-authored-by: Deependu <deependujha21@gmail.com>