Fix Qwen2-VL image/video processor legacy size field handling
Author: Isotr0py
Pull request: yes

# What does this PR do?

- Fixes https://github.com/vllm-project/vllm/issues/15614#issuecomment-3097032491

Some qwen2-vl based model like `ByteDance-Seed/UI-TARS-2B-SFT` has legacy `size` field in `preprocessor.json` like: 
```
"size": {
    "max_pixels": 2116800,
    "min_pixels": 3136
}
```
Which will raise `ValueError: size must contain 'shortest_edge' and 'longest_edge' keys.` after v4.52

This PR adds backwards compatibility for this kind of `size` field.

**Code to reproduce**
```python3
from transformers import AutoProcessor

processor = AutoProcessor.from_pretrained("ByteDance-Seed/UI-TARS-2B-SFT")
print(processor)

processor = AutoProcessor.from_pretrained("ByteDance-Seed/UI-TARS-2B-SFT", use_fast=True)
print(processor)
```


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Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_39570). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

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

run-slow: qwen2_vl

This is not in a hurry, so let's wait #39591 merged instead!

Just seeing this sorry! It wasn't completely fixed in #39591 but it should be now with the latest commit