Pass correct num_items_in_batch value into the training_step function

[hiyouga]

What does this PR do?

This PR follows #34511 #34915 and partially reverts #35121 . We want to handle the loss calculation correctly for models that don't accept loss_kwargs when gradient accumulation was enabled.

In #35121 , the author always passes num_items_in_batch into the training_step function, which makes the logic of the following lines invalid, and consequently leads to incorrect loss calculation.

transformers/src/transformers/trainer.py

Lines 3701 to 3707 in 3b0a94e

	# Finally we need to normalize the loss for reporting
	if num_items_in_batch is None:
	loss = loss / self.args.gradient_accumulation_steps
	self.accelerator.backward(loss, **kwargs)
	return loss.detach()

Currently, there are still many models that don't accept loss_kwargs (such as Qwen2-VL). For these models, we need to set the num_items_in_batch parameter in training_step function to None, so as to scale the loss correctly in training.

However, we've found that since transformers 4.46.0, the loss of the models that don't accept loss_kwargs hasn't been calculated correctly!

Specifically, in transformers 4.46.0-4.46.1, the loss was wrongly multiplied by the gradient accumulation steps, resulting in a large loss value (see https://twitter.com/TheZachMueller/status/1851677628656423347 ).

https://github.com/huggingface/transformers/blob/v4.46.0/src/transformers/trainer.py#L3605

transformers/src/transformers/trainer.py

Lines 3605 to 3608 in c2820c9

	loss *= self.args.gradient_accumulation_steps
	self.accelerator.backward(loss, **kwargs)
	return loss.detach() / self.args.gradient_accumulation_steps

In transformers 4.46.2-4.47.1, the loss was scaled after the backward pass, which also led to larger gradients (also mentioned in #35207 ).

https://github.com/huggingface/transformers/blob/v4.47.1/src/transformers/trainer.py#L3689

transformers/src/transformers/trainer.py

Lines 3687 to 3691 in 241c04d

	self.accelerator.backward(loss, **kwargs)
	# Finally we need to normalize the loss for reporting
	if num_items_in_batch is None:
	return loss.detach() / self.args.gradient_accumulation_steps
	return loss.detach()

In the latest version, due to the issues in this PR, the loss isn't scaled correctly either, and the final loss remains large. We hope the maintainers can attach importance to this problem and look forward to a timely fix.

We also conducted a group of experiments to verify the above conclusion. We trained the Qwen2-VL model (which does not accept loss_kwargs) using transformers 4.45.2, the main branch (5c75087), and the main branch after the PR fix. We can find that both version 4.45.2 and the fixed version have reasonable loss values and grad norms, while the main branch does not.

transformers version	micro batchsize	grad accumulation steps	avg loss	avg grad norm
4.45.2	4	2	1.17	0.32
4.45.2	1	8	0.99	0.43
main (4.48.0.dev0)	4	2	2.34	0.65
main (4.48.0.dev0)	1	8	7.93	3.52
main + this PR	4	2	1.17	0.32
main + this PR	1	8	0.99	0.43

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline,
  Pull Request section?
• Was this discussed/approved via a Github issue or the forum? Please add a link
  to it if that's the case.
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  documentation guidelines, and
  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

@ArthurZucker @muellerzr

[hiyouga]

The test cases have almost passed, except for a few that encountered a timeout error.

pipelines_tf - Failed
tests_exotic_models - Success
collection_job - Success
tests_onnx - Success
tests_torch_and_tf - Success
tests_torch - Success
tests_hub - Success
tests_processors - Success
examples_torch - Success
tests_custom_tokenizers - Success
tests_generate - Success
tests_tf - Success
tests_non_model - Success
examples_tensorflow - Success
tests_torch_and_flax - Success
tests_tokenization - Success
pipelines_torch - Success