Correctly support resuming from checkpoint with a dataset without length

[muupan]

What does this PR do?

There is an inconsistency in Trainer's behavior between training from scratch and resuming from checkpoint when the given dataset has no length like datasets.IterableDataset. For a reproducible example, see #26413 (comment) . This PR fixes the inconsistency by correctly supporting resuming from checkpoint with such a dataset.

Fixes #26413

Current behavior

When training starts with a dataset without length, Trainer assumes one epoch is equal to max_steps steps and tries to train for that many steps. There are two possible scenarios.

• A. If the dataset yields enough samples, the training finishes precisely after one epoch.
• B. If the dataset raises StopIteration before yielding samples enough for max_steps steps, Trainer increments the current epoch and re-iterate the dataset.

When resuming from a checkpoint, Trainer simply skips the first batches until global_step of the checkpoint. In scenario A, there is no problem. In scenario B, the dataset raises StopIteration during the skipping, but Trainer does not re-iterate the dataset. Instead, it just finishes training with a warning. This is inconsistent from what happens in training from scratch, and it contradicts with what the documents about max_steps says:

transformers/src/transformers/training_args.py

Lines 301 to 304 in ac5a055

	max_steps (`int`, *optional*, defaults to -1):
	If set to a positive number, the total number of training steps to perform. Overrides `num_train_epochs`.
	For a finite dataset, training is reiterated through the dataset (if all data is exhausted) until
	`max_steps` is reached.

Solution

This PR modifies the skipping behavior so that Trainer now re-iterates the dataset until it catches up global_step. A caveat is that it does not support the ignore_data_skip option, as Trainer does not know what epoch to start from. I am also concerned that the logic is becoming too complicated.

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• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
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@ArthurZucker

[LysandreJik]

Very impressive PR @muupan!

I'm pinging @muellerzr and @SunMarc to take a look; Zach is off for a few weeks and will take a look as soon as he's back, thank you for your patience 🙏

[SunMarc]

Thanks for the PR @muupan ! We will review it shortly. There is a new feature in accelerate that enable you to use a stateful dataloader, so that we don't need to iterate to resume a training. Feel free to give it a try, note that it is a very experimental support for now.

[muupan]

It seems like the code got broken after rebasing with main, where #34198 renamed the variable epoch_iterator. I will fix.

[SunMarc]

Let us knew when it is done !