Unify DINOv3 fine-tuning backbone loading

[guarin]

What has changed and why?

• Unify DINOv3 fine-tuning backbone loading
• Fix bug when loading dinov3/vitt16-ltdetr which loaded dinov3/vitt16-notpretrained-ltdetr instead

How has it been tested?

• Manually

Did you update CHANGELOG.md?

• Yes
• Not needed (internal change)

Did you update the documentation?

• Yes
• Not needed (internal change without effects for user)

[guarin]

/review

[Copilot]

Pull request overview

This PR unifies DINOv3 fine-tuning backbone loading across LTDETR object detection and EoMT segmentation task models, and fixes an issue where certain LTDETR backbones/checkpoints would not load the intended weights.

Changes:

• Route backbone weight handling through DINOV3_PACKAGE.get_model(...) consistently (instead of per-task special casing).
• Adjust DINOv3 package logic to set pretrained=True when explicit weights are provided.
• Add logging around checkpoint loading and document the fix in CHANGELOG.md.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/lightly_train/_task_models/dinov3_ltdetr_object_detection/task_model.py	Uses unified DINOV3_PACKAGE.get_model backbone loading and removes local path existence branching.
src/lightly_train/_task_models/dinov3_eomt_semantic_segmentation/task_model.py	Switches backbone loading to the package API and removes custom weight-loading helper.
src/lightly_train/_task_models/dinov3_eomt_panoptic_segmentation/task_model.py	Same unification as semantic segmentation; removes custom weight-loading helper.
src/lightly_train/_task_models/dinov3_eomt_instance_segmentation/task_model.py	Same unification as semantic segmentation; removes custom weight-loading helper.
src/lightly_train/_models/dinov3/dinov3_src/hub/backbones.py	Logs the resolved checkpoint URL used for weight loading.
src/lightly_train/_models/dinov3/dinov3_package.py	Ensures pretrained is enabled when explicit weights are provided and load_weights=True.
CHANGELOG.md	Adds a “Fixed” entry describing the LTDETR backbone-loading bug fix.

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