feat: add padding_side param in DataCollatorWithPadding
Author: ddz5431
Pull request: yes

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# 📌 What does this PR do?
This PR **features and enhances the handling of `padding_side`** in `DataCollatorWithPadding` by ensuring that `padding_side` is **properly passed to the tokenizer’s `.pad()` method** instead of modifying `self.tokenizer.padding_side` globally.

### 🔹 Why is this change necessary?
- Previously, `padding_side` **was not explicitly passed** to `.pad()`, meaning users had to **manually modify `tokenizer.padding_side`**, which caused unintended global side effects.
- Some models (e.g., **GPT-2, LLAMA**) require **left-padding**, while others (e.g., **BERT, T5**) require **right-padding**.
- Without this feature, **users could not dynamically set `padding_side` per batch**, making it harder to handle mixed padding requirements.

### 🔹 What does this feat do?
- It **ensures `padding_side` is explicitly passed** as an argument to `.pad()`, making it configurable **per batch** instead of requiring global modifications.
- This approach **removes unintended global side effects** and allows **greater flexibility** in tokenization pipelines.

## Before submitting
- [x] 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?
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      to it if that's the case.
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      [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?


## Who can review?

Anyone in the community is free to review the PR once the tests have passed. The following contributors might be interested:

- **Tokenizers & Data Collation:** @ArthurZucker  
- **Trainer & Model Processing:** @muellerzr  
- **General Library Review:** @Rocketknight1  
- **Documentation Review:** @stevhliu  

If no one reviews this PR after a week, feel free to tag these members in a comment to request a review.

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