[Train] Add a barrier in RayTrainReportCallback to ensure synchronous reporting.

[woshiyyya]

Why are these changes needed?

ray.train.report ensures that all workers enter at the same time, but there are no barriers preventing all workers from exiting at the same time. This could lead to an FileNotFoundError in RayTrainReportCallback, where local_rank_0 worker exit earlier, deleted the ckpt folder, while other workers are still uploading it.

This PR fixed this bug by adding a barrier call right after ray.train.report.

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