refactor: ♻️ update YOLO12 example doc and code

[onuralpszr]

• Update requirements.txt for YOLO12 dependencies
• Add .gitignore for YOLO12 model files
• Update README.md to improve clarity and formatting
• Replace de_parallel with unwrap_model in export_and_validate.py

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/14771

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❌ 1 New Failure

As of commit 75e46d7 with merge base 3557edf ():

NEW FAILURE - The following job has failed:

• pull / test-samsung-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t a7734221cfb67b485d1739baad61aeae991b026bd3524510036eb24414679f7e /exec failed with exit code 1

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[mergennachin]

cc @daniil-lyakhov fyi

[onuralpszr]

cc @ambitious-octopus can you test please (benchmarks please)

[digantdesai]

Thanks!

[cavusmustafa]

The update works with openvino backend as expected. (verified with ultralytics version 8.3.196)

[ambitious-octopus]

Thanks for testing @cavusmustafa!

[digantdesai]

Curious, did you test this on XNNPACK backend as well?

[digantdesai]

ok I see - #11523 :)