Fix Sam3Processor missing input_boxes_labels for padded None entries

[Kash6]

When input_boxes contains a mix of None and real box entries (e.g. input_boxes=[None, [[x1,y1,x2,y2]]]), the processor pads None entries with [-10,-10,0,0] but does not generate corresponding input_boxes_labels. The model's geometry encoder then treats all boxes as valid (box_mask=ones), including the padded ones, causing different behavior compared to input_boxes=None.

Generate default input_boxes_labels when not provided by the user: None entries get label -10 (the pad value) so the model masks them out, real entries get label 1 (positive). User-provided labels are preserved unchanged.

Adds processor tests covering mixed None/real batches, all-real batches, no-boxes, and user-provided label preservation.

What does this PR do?

Fixes #45059

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@yonigozlan @molbap @NielsRogge @zucchini-nlp

[Rocketknight1]

cc @yonigozlan @molbap @NielsRogge!

[Kash6]

Friendly ping @yonigozlan @NielsRogge @molbap @Rocketknight1, this is a small processor fix (one new method and 4 tests) for the batched input_boxes inconsistency in Sam3Processor. CI quality checks all pass; the failing jobs are pre-existing flaky tests on main. Happy to address any feedback.

[yonigozlan]

Hello @Kash6 ! Thanks a lot for working on this and fixing this bug. I just made a few simplifications/standardization, and we're ready to merge!

[github-actions]

[For maintainers] Suggested jobs to run (before merge)

run-slow: sam3

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