[Train][transformers] Skip reporting checkpoint when get_latest_checkpoint returns None.

[woshiyyya]

Why are these changes needed?

An OSS users found that RayTrainReportCallback fails to copy checkpoint while doing multi-node distributed training: #42926

Previously, transformers Trainer first creates a tmp dir for all workers to save checkpoint files, then renames the tmp-checkpoint-* dir to checkpoint-* on all workers. They recently introduced a behavior change, which forces it to only rename on global-rank-0 worker (if save_on_all_nodes=False). Therefore, except for node of rank 0 worker, the remaining nodes still keep the tmp-checkpoint-* dir around, and returns None in transformers.trainer.get_last_checkpoint since no checkpoint-* folders are found in the the args.output_dir (see here).

This PR modifies ray.train.transformers.RayTrainReportCallback to only report checkpoint when transformers.trainer.get_latest_checkpoint returns a valid path. This is to address a behavior change made in transformers starting in 4.37.0 where only the global rank 0 worker will rename the temporary checkpoint directory so that it is found by this get_latest_checkpoint function.

Update: This feature is introduced in the lastest transformers version, but we will not update the package version to avoid breaking other tests.

Related issue number

Closes: #42926

Checks

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[justinvyu]

Thanks!

[viirya]

Hi, thanks for fixing this. I also encountered this bug today with ray 2.9.2. When I tried to fix it and just found it is already fixed in latest branch.

But I just checked ray 2.9.3 which is released 2 days ago. This fix seems not included there. Actually this commit looks like merged in 2/13, it should be in the time range of ray 2.9.2 - ray 2.9.3 (ray-2.9.2...ray-2.9.3, where latest commit is 2/15).

Any idea why this fix isn't included and when a new release will include this fix? Thanks.

[woshiyyya]

@viirya This will be included in 2.10 (release in the next month). You can use our nightly build if you want to include this fix asap.

[viirya]

Got it. Thank you @woshiyyya