Fix: Set clean_up_tokenization_spaces

[Aznix07]

What does this PR do?

This PR fixes a regression where clean_up_tokenization_spaces default was changed from True to False in v5.0.0rc1, breaking backward compatibility with v4.x.

Problem:

In transformers v5.0.0rc1, tokenizers no longer clean up spaces before punctuation by default:

from transformers import AutoTokenizer

tokenizer = AutoTokenizer.from_pretrained("google/gemma-2-9b-it")
text_input = "beginning , and"    
ids = tokenizer(text_input).input_ids
decoded = tokenizer.decode(ids, skip_special_tokens=True)

# v4.57.3:   "beginning, and"  (spaces cleaned)
# v5.0.0rc1: "beginning , and" (spaces NOT cleaned)

Solution:

Changed the default value in tokenization_utils_base.py (line 1420):

- self.clean_up_tokenization_spaces = kwargs.pop("clean_up_tokenization_spaces", False)
+ self.clean_up_tokenization_spaces = kwargs.pop("clean_up_tokenization_spaces", True)

Safety:
This change only affects post-processing of decoded text, NOT:

• Token IDs sent to the model (verified unchanged)
• Model predictions
• Training behavior

Fixes #42898

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Who can review?

@ArthurZucker @itazap

[github-actions]

View the CircleCI Test Summary for this PR:

https://huggingface.co/spaces/transformers-community/circle-ci-viz?pr=42900&sha=aeba93

[Aznix07]

I tried fixing this on main by changing the default clean_up_tokenization_spaces to True, but tokenization tests (e.g. Wav2Vec2, CLVP) expect the default to remain False in v5 and fail accordingly.

From the issue description, the real problem seems to be in the new v5 tokenization backends (TokenizersBackend / PythonBackend), where decode(..., clean_up_tokenization_spaces=True) doesn’t call a default clean_up_tokenization implementation.

Could you confirm:

• Should the fix target the v5 branch instead of main?
• Is the expected behavior in v5: default clean_up_tokenization_spaces=False globally, but decode(..., clean_up_tokenization_spaces=True) must always apply cleanup logic?

I’d be happy to move my work to the v5 branch and implement this properly once I know which files/classes to target.