fix(models): Fix LayoutLMv2 NER crash and broken batched truncation/padding

[harshaljanjani]

What does this PR do?

The following issues were identified and fixed in this PR:

→ The NER/token classification issue and the downstream bug uncovered in the batched preprocessing use case with LayoutLMv2Tokenizer.
→ Reasoning: The NER use case makes it apparent that the error is hit at this line in LayoutLMv2Tokenizer, which is missing self.only_label_first_subword. PretrainedTokenizerBase doesn't create self.only_label_first_subword either, so this must be added along with the other custom attributes, and this directly resolved the NER use case as shown in the screenshot.
→ For the second fix; any padding="max_length" or truncation=True call without an explicit max_length arg compares self.model_max_length > LARGE_INTEGER (1e20), which in this case evaluates to True (since model_max_length falls back to VERY_LARGE_INTEGER), and both get translated into no-ops. Sequences in the batch are of different lengths and can't be tensorized, and the misleading ValueError tells the user to add padding=True and truncation=True, but they already did? The fix restores model_max_length=512. I confirmed both the base and large model configs have max_position_embeddings=512, so 512 as a default is correct, and followed the same pattern as MarianTokenizer and TapasTokenizer :)

Originally removed in #42894, just wanted to double-check if this was intentional; happy to adjust this fix if I’ve missed something :)

Fixes #44186.

Before both fixes applied:

Attribute fix resolves NER; batched use case still fails (feel free to cross-check; the errors are reproducible):

After both fixes applied; NER + batched use case work (feel free to cross-check):

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[github-actions]

[For maintainers] Suggested jobs to run (before merge)

run-slow: layoutlmv2

[zucchini-nlp]

Thanks for the detailed explanation, lgtm! Btw, @ArthurZucker , I think we should raise a warning/error asking to pass a max_length arg when model_max_length > VERY_LARGE_INT because the current error message is misleading

[zucchini-nlp]

run-slow: layoutlmv2

[github-actions]

Workflow Run ⚙️

This comment contains run-slow, running the specified jobs:

models: ["models/layoutlmv2"]
quantizations: []

[HuggingFaceDocBuilderDev]

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[github-actions]

CI Results

Workflow Run ⚙️

Commit Info

Context	Commit	Description
RUN	d4ac02a8	workflow commit (merge commit)
PR	ff05e228	branch commit (from PR)
main	df1cd3a7	base commit (on main)

✅ No failing test specific to this PR 🎉 👏 !