fix(testing): Fix LayoutXLM tokenization test and LightOnOCR SDPA flash test failures on main CI

[harshaljanjani]

What does this PR do?

The following failing tests were identified and fixed in this PR:

→ LayoutXLM: This PR (rm slow tokenizers) changed models/auto/tokenization_auto.py to map configs to single tokenizer classes instead of (slow, fast) tuples. It did touch tests/test_tokenization_common.py; having said that, it didn't update the merge_model_tokenizer_mappings fn so the [0] and [1] subscripts fail now.
→ LightOnOCR: This commit (Add LightOnOCR model implementation) added the LightOnOCR model which internally uses Pixtral's vision encoder. The skip list for test_sdpa_can_dispatch_on_flash already included "pixtral" (added on Jun 27, 2025), but LightOnOCR was not added to the same list despite having identical behavior (internally generates a block attention mask incompatible with Flash Attention), this should fix that :)

cc: @Rocketknight1

→ After: Both tests correctly skip gracefully (ref screenshots). merge_model_tokenizer_mappings should handle V5's single-class tokenizer mapping, and lighton_ocr should be skipped in test_sdpa_can_dispatch_on_flash alongside pixtral!

CI Failures:

Before the fix (feel free to cross-check; these errors are reproducible):

After the fix (feel free to cross-check):

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Did you fix any necessary existing tests?

[Rocketknight1]

cc @ArthurZucker @itazap to do a quick review since this is related to v5's new tokenizer mappings!

[HuggingFaceDocBuilderDev]

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[itazap]

run-slow

[harshaljanjani]

Just checking in to see if there have been any updates here :)

[itazap]

run-slow: auto, layoutxlm, lightonocr

[github-actions]

Workflow Run ⚙️

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

models: ["models/auto", "models/layoutxlm"]
quantizations: []

[github-actions]

CI Results

Workflow Run ⚙️

Commit Info

Context	Commit	Description
RUN	99946f2a	workflow commit (merge commit)
PR	1b982539	branch commit (from PR)
main	df1cd3a7	base commit (on main)

✅ No failing test specific to this PR 🎉 👏 !