Repository: huggingface/transformers
Pull request: GemmaTokenizer: remove redundant whitespace pre-tokenizer

# What does this PR do?

This PR removes a redundant whitespace pre-tokenizer from GemmaTokenizer and adds a regression test to ensure behavior remains unchanged.

Specifically, GemmaTokenizer already normalizes literal spaces (" ") to the SentencePiece underline character ("▁"). As a result, using a pre_tokenizer=Split(" ", "merged_with_previous") is a no-op because no literal spaces remain after normalization.

This PR:
	•	Removes the redundant pre_tokenizer configuration from GemmaTokenizer
	•	Adds a regression test confirming that disabling the pre-tokenizer does not change tokenize() or encode() outputs
	•	Cleans up an unused import

There is no behavioral change to tokenization outputs.



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Fixes #43062


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#create-a-pull-request),
      Pull Request section?
- [x] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
      Linked issue: https://github.com/huggingface/transformers/issues/43062
- [x] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
     Inline code comment added explaining why the pre-tokenizer is redundant.
- [x] Did you write any new necessary tests?


## Who can review?
@ArthurZucker @itazap
Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
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Models:

- text models: @ArthurZucker @Cyrilvallez
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- generate: @zucchini-nlp (visual-language models) or @gante (all others)
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Changed files:
filename: src/transformers/models/gemma/tokenization_gemma.py
patch: @@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from tokenizers import Tokenizer, decoders, normalizers, pre_tokenizers
+from tokenizers import Tokenizer, decoders, normalizers
 from tokenizers.models import BPE
 
 from ...tokenization_utils_tokenizers import TokenizersBackend
@@ -93,7 +93,6 @@ def __init__(
             [decoders.Replace("▁", " "), decoders.ByteFallback(), decoders.Fuse()]
         )
         self._tokenizer.normalizer = normalizers.Replace(" ", "▁")
-        self._tokenizer.pre_tokenizer = pre_tokenizers.Split(" ", "merged_with_previous")
         super().__init__(
             unk_token=unk_token,
             bos_token=bos_token,