Fix grad accum arbitrary value

[IlyasMoutawwakil]

What does this PR do?

The test for the "brokenness" of gradient accumulation loss uses an arbitrary value to check if not passing "num_items_in_batch" results in a loss different from normal training loss. This value got smaller after I made some changes to the test (mostly trying to make it more understandable).
See changes in https://github.com/huggingface/transformers/pull/36424/files#diff-ac9b69e204d41f7ddf23e8bd5b53a2a0ffa64c4784101abeef3b5d4c14342656R823-R905

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[IlyasMoutawwakil]

@muellerzr @Cyrilvallez

[HuggingFaceDocBuilderDev]

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[SunMarc]

Works for me !

[Cyrilvallez]

Alright, LGTM! Thanks a lot!