make num_items_in_batch optional in compute_loss_func

[SunMarc]

What does this PR do?

Fixes #36331

This PR fixes the breaking change that we have with custom loss function as we introduced num_items_in_batch.

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[ruidazeng]

Is there a way to make it work even if we don't specify compute_loss_func.

[techkang]

I think the num_items_in_batch argument should be mandatory because leaving it as None when compute_loss_func is set could trigger the GA loss bug. A better way to fix issue #36331 might be to add more specific documentation for compute_loss_func.