🏗️ Add test for training with multiple dataloader workers and update worker initialization for compatibility with transformers 4.52.0

[qgallouedec]

What does this PR do?

This PR fixes two issues related to distributed setting:

• Transformers breaking change, seed_worker now requires two new arguments: num_workers and rank, see update seed_worker to set seed based on worker_id and rank transformers#37980
• And pickling error because of collator being defined inside GRPOTrainer, see Data collator not found during pickling with trl 0.18.1 and pytorch 2.7 #3567

Fixes #2779 #2979 #3567

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[Tavish9]

For spawn method (the default on Windows and macOS), method requires that all objects (including functions) passed between processes need to be serialized (pickled).

Pickle only serializes objects that are defined at the top-level of a module.

So we need to move local function outwards to support MultiProcessingDataLoader under spawn context

[Tavish9]

please also link to

• ORPOTrainer crashes due to pickling failure if dataloader_num_workers > 0 #2779
• AttributeError: Can't pickle local object 'GRPOTrainer.__init__.<locals>.data_collator' #2979

same as

• Data collator not found during pickling with trl 0.18.1 and pytorch 2.7 #3567

[qgallouedec]

Thanks for the pointers @Tavish9! Added in solved issues

[shirinyamani]

Suggested change

	"""Do we really need docs for this?"""
	"""GRPO does not need data_collator, to avoid crash, this simple function will be used as data_collator when initializing the GRPOTrainer"""

[qgallouedec]

we already document it below, I think it's not necessary

[shirinyamani]

The identity method looks like what we had as data_collator method, so do you think identity name is more clear?

def data_collator(features):  # No data collation is needed in GRPO
    return features

also now that this is defines in global scope, I'm curios to see does this solve issue #3567

[qgallouedec]

I prefer to keep the name identity because it reflects what the function does regardless of where it's used (even if we only use it as data collator). Naming based on behavior rather than context keeps the function general.

I'm curios to see does this solve issue

@Tavish9 gave some clue on this. Basically we need the class to be pickable, which is not the case when function are defined within methods

[Tavish9]

@qgallouedec Since this pr fixed two things: pickle and transformers update, it should also be linked to the following issues:

• GRPOTrainer crashs with dataloader_num_workers >= 1 #3544