fix(models): Rename fpn_position_encoding to fpn_position_embeddings
Author: lllangWV
Pull request: yes

# What does this PR do?


The dataclass output attribute was named `fpn_position_encoding` but code in `_prepare_vision_features` was accessing `fpn_position_embeddings`, causing an AttributeError. Renamed to `fpn_position_embeddings` across all SAM2, SAM3, and EdgeTAM models for consistency with transformers naming conventions.

Affected models: sam2, sam2_video, sam3, sam3_tracker, sam3_tracker_video, sam3_video, edgetam, edgetam_video

Maybe the other changes where unnecessary, but I like to have consistent naming scheme through similar modules.

I could have just modified 

transformers/models/sam3_tracker_video/modeling_sam3_tracker_video.py", line 1912  to 
```python
vision_pos_embeds = image_outputs.fpn_position_embeddings 

->

vision_pos_embeds = image_outputs.fpn_position_encodings
```


If you all just want the minimal change I can go back and undo the other changes

## Before submitting
- [ ] 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?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [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).
- I ran `doc-builder build transformers docs/source/en --build_dir ~/tmp/test-build` Nothing seemed to change. I think the docs for this get automatically updated.

```bash
Building docs for transformers docs/source/en /home/lllang/tmp/test-build/transformers/main/en
Building the MDX files:  13%|████████████████████████                                                                                                                                                                | 81/618 [00:00<00:00, 797.15it/s]use_kernel_func_from_hub is not available in the installed kernels version. Please upgrade kernels to use this feature.
Building the MDX files: 100%|████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████| 618/618 [00:47<00:00, 13.12it/s]
Resolving internal links: 100%|█████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████| 605/605 [00:01<00:00, 584.87it/s]

```

- [ x] Did you write any new necessary tests? 
  - Due to the name change, a test had to be modified `tests/models/sam3/test_modeling_sam3.py`


## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

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Models:

- text models: @ArthurZucker @Cyrilvallez
- vision models: @yonigozlan @molbap
- audio models: @eustlb @ebezzam @vasqu
- multimodal models: @zucchini-nlp
- graph models: @clefourrier

Library:

- generate: @zucchini-nlp (visual-language models) or @gante (all others)
- continuous batching: @remi-or @ArthurZucker @McPatate
- pipelines: @Rocketknight1
- tokenizers: @ArthurZucker and @itazap
- trainer: @SunMarc
- attention: @vasqu @ArthurZucker @CyrilVallez
- model loading (from pretrained, etc): @CyrilVallez
- distributed: @3outeille @ArthurZucker
- CIs: @ydshieh

Integrations:

- ray/raytune: @richardliaw, @amogkam
- Big Model Inference: @SunMarc
- quantization: @SunMarc @MekkCyber
- kernels: @MekkCyber @drbh
- peft: @BenjaminBossan @githubnemo

Devices/Backends:

- AMD ROCm: @ivarflakstad
- Intel XPU: @IlyasMoutawwakil
- Ascend NPU: @ivarflakstad 

Documentation: @stevhliu

Research projects are not maintained and should be taken as is.

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Comments:
**[For maintainers]** Suggested jobs to run (before merge)

run-slow: edgetam, edgetam_video, sam2, sam2_video, sam3, sam3_tracker, sam3_tracker_video, sam3_video

cc @nielsrogge @yonigozlan @molbap 

Hello!

This was taken care of with #43487, which instead renames `fpn_position_embeddings` (which was erroneously introduced in #42564 3 days ago) to `fpn_position_encoding`, which was originally intended. Thank you for your PR, but the issue has now been resolved.

- Tom Aarsen