Make size_dict conversion logs clearer
# What does this PR do?

* Tidies up logic for converting `size` parameter to the expected dictionary format for image processors.
* Adds `param_name` as a flag so logs reflect the variable being updated e.g. `crop_size` versus `size`

Address part of #20185  - trying to make the logs clearer. 

## Before submitting
- [ ] 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.
- [ ] 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?
@sgugger @patrickvonplaten  The quickest and easiest remedy I had was adding a `param_name` flag for the `get_size_dict` function. This way, if an image processor has both `size` and `crop_size` variables being updated, the logs reflect the parameter being changed. However, it's a bit of a dirty trick. LMK if you have an alternative suggestion. 

<|||||>The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_20205). All of your documentation changes will be reflected on that endpoint.<|||||>The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_20205). All of your documentation changes will be reflected on that endpoint.<|||||>Thanks!