Add auto_docstring decorator to Sam3ImageProcessorFast
Author: anthonyylee
Pull request: yes

[Sam3ImageProcessorFast](https://huggingface.co/docs/transformers/main/en/model_doc/sam3#transformers.Sam3ImageProcessorFast) inherits from [Sam2ImageProcessorFast](https://huggingface.co/docs/transformers/main/en/model_doc/sam2#transformers.Sam2ImageProcessorFast), however the docstring decorator is left out and only the SAM2 decorated `preprocess` method is showing up in Sam3ImageProcessorFast's documentation.

# What does this PR do?

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Improved the SAM3 model documentation.


## 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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      [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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@yonigozlan
@ronghanghu

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Comments:
**[For maintainers]** Suggested jobs to run (before merge)

run-slow: sam3