fix check for loss_kwargs/GA

[winglian]

What does this PR do?

#34915 didn't quite do what was intended and seems to break packing. Since loss_kwargs was replaced with num_items_in_batch, we should probably be checking for that. This fixes tests on our end that runs a fine-tune with packing and validates the loss. Without this fix, the loss is ~3x larger than it should be with packing.

Fixes # (issue)

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[winglian]

I effed this one up