rewrite main method in Qwen2, making it more clear
Author: ailunc
Pull request: yes

# What does this PR do?

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Fixes # (issue)
[Issue #36667](https://github.com/huggingface/transformers/issues/36667)

## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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      Pull Request section?
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      to it if that's the case. [Link](https://github.com/huggingface/transformers/issues/36667)
- [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).
- [ ] Did you write any new necessary tests?


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Models:

- text models: @ArthurZucker
- vision models: @amyeroberts, @qubvel
- speech models: @eustlb
- graph models: @clefourrier

Library:

- flax: @gante and @Rocketknight1
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- chat templates: @Rocketknight1

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Comments:
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hi @ailunc, this is a lot cleaner, thank you! Can you `pip install transformers[quality]` and then `make style` to ensure that the formatting is fixed up and then ping me to merge it?

> hi @ailunc, this is a lot cleaner, thank you! Can you `pip install transformers[quality]` and then `make style` to ensure that the formatting is fixed up and then ping me to merge it?

hi @Rocketknight1 ! I ran the lines and everything looks good.