Fix local_files_only tokenizer fallback when tokenizer files are missing (Issue 45538)

[Brianzhengca]

What does this PR do?

Fixes #45538

Root Cause

from_pretrained(..., local_files_only=True) let missing tokenizer files resolve to None and still proceeded into _from_pretrained(...). That allowed a stub tokenizer to initialize instead of raising, which led to the bogus huge model_max_length.

Describe the Fix

The fix adds a fail-closed check in PreTrainedTokenizerBase.from_pretrained.

After resolving files, it now checks whether all real tokenizer assets for that class resolved to None.
If they did, it raises the existing Can't load tokenizer... OSError instead of continuing into
_from_pretrained(...) and constructing a stub tokenizer

Local Tests

• Check with BertTokenizer and CLIPTokenizer using a nonexistent model id and
  local_files_only=True.
  Before the fix, they loaded unexpectedly with the huge fallback max length.
  After the fix, they raised the expected OSError.

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  to it if that's the case. (CLIPTokenizer uses 10**30 as model_max_length #45538)
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@ArthurZucker @Cyrilvallez

[ArthurZucker]

Thanks.
Failing tests are related:

FAILED tests/models/auto/test_tokenization_auto.py::AutoTokenizerTest::test_custom_tokenizer_with_mismatched_tokenizer_class - OSError: Can't load tokenizer for 'hf-internal-testing/test_unregistered_dynamic'. If you were trying to load it from 'https://huggingface.co/models', make sure you don't have a local directory with the same name. Otherwise, make sure 'hf-internal-testing/test_unregistered_dynamic' is the correct path to a directory containing all relevant files for a NopTokenizer tokenizer.
FAILED tests/models/auto/test_tokenization_auto.py::AutoTokenizerTest::test_init_tokenizer_with_trust - OSError: Can't load tokenizer for 'hf-internal-testing/test_unregistered_dynamic'. If you were trying to load it from 'https://huggingface.co/models', make sure you don't have a local directory with the same name. Otherwise, make sure 'hf-internal-testing/test_unregistered_dynamic' is the correct path to a directory containing all relevant files for a NopTokenizer tokenizer.

and DIA ones, can you please fix them accordingly? 🤗

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[Brianzhengca]

@ArthurZucker, just fixed my code for the tests. Please take a look when you have time. Thank you!