GemmaTokenizer: remove redundant whitespace pre-tokenizer
Author: vaibhav-research
Pull request: yes

# 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.



<!--
Congratulations! You've made it this far! You're not quite done yet though.

Once merged, your PR is going to appear in the release notes with the title you set, so make sure it's a great title that fully reflects the extent of your awesome contribution.

Then, please replace this with a description of the change and which issue is fixed (if applicable). Please also include relevant motivation and context. List any dependencies (if any) that are required for this change.

Once you're done, someone will review your PR shortly (see the section "Who can review?" below to tag some potential reviewers). They may suggest changes to make the code even better. If no one reviewed your PR after a week has passed, don't hesitate to post a new comment @-mentioning the same persons---sometimes notifications get lost.
-->

<!-- Remove if not applicable -->

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
members/contributors who may be interested in your PR.

<!-- Your PR will be replied to more quickly if you can figure out the right person to tag with @

 If you know how to use git blame, that is the easiest way, otherwise, here is a rough guide of **who to tag**.
 Please tag fewer than 3 people.

Models:

- text models: @ArthurZucker @Cyrilvallez
- vision models: @yonigozlan @molbap
- audio models: @eustlb @ebezzam @vasqu
- multimodal models: @zucchini-nlp
- graph models: @clefourrier

Library:

- generate: @zucchini-nlp (visual-language models) or @gante (all others)
- continuous batching: @remi-or @ArthurZucker @McPatate
- pipelines: @Rocketknight1
- tokenizers: @ArthurZucker and @itazap
- trainer: @SunMarc
- attention: @vasqu @ArthurZucker @CyrilVallez
- model loading (from pretrained, etc): @CyrilVallez
- distributed: @3outeille @ArthurZucker
- CIs: @ydshieh

Integrations:

- ray/raytune: @richardliaw, @amogkam
- Big Model Inference: @SunMarc
- quantization: @SunMarc @MekkCyber
- kernels: @MekkCyber @drbh
- peft: @BenjaminBossan @githubnemo

Devices/Backends:

- AMD ROCm: @ivarflakstad
- Intel XPU: @IlyasMoutawwakil
- Ascend NPU: @ivarflakstad 

Documentation: @stevhliu

Research projects are not maintained and should be taken as is.

 -->


Comments:
Sorry for the tag. if I can get some eyes here for review that will be great. cc @ArthurZucker 
Thanks

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

run-slow: gemma

thanks @itazap for reviewing, I have removed the comment as well.

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_43106). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

run-slow: gemma

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

models: ["models/gemma"]
quantizations: []

## CI Results
[Workflow Run ⚙️](https://github.com/huggingface/transformers/actions/runs/20819661457)

✅ No failing test specific to this PR 🎉 !