Trainer: Pass num_items_in_batch to compute_loss in prediction_step

[pramodith]

What does this PR do?

Ensures that num_items_in_batch is passed to the compute_loss function in the prediction_step to ensure that loss is calculated the same way both at train and eval time.

Fixes #41108

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@SunMarc

[pramodith]

I think the test case could be better open to hearing suggestions to add tests (for multi-gpu?) or modify the test.

[SunMarc]

LGTM ! Thanks for this nice PR !

[SunMarc]

Hmmm these tests are still failing: tests/trainer/test_trainer.py::TrainerIntegrationTest::test_evaluate_with_jit, can you quickly check why ? I guess the simplest solution would be to check if we have self.args.jit_mode_eval or not. Btw, we should probably deprecate this arg also

[HuggingFaceDocBuilderDev]

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[pramodith]

Hmmm these tests are still failing: tests/trainer/test_trainer.py::TrainerIntegrationTest::test_evaluate_with_jit, can you quickly check why ? I guess the simplest solution would be to check if we have self.args.jit_mode_eval or not. Btw, we should probably deprecate this arg also

Will take a look in a few hours once I'm off work.