Fix masks_to_boxes for empty masks and vectorize for performance

[raimbekovm]

Fixes #9346

masks_to_boxes raises RuntimeError when a mask is all zeros because torch.min/max on empty tensor requires explicit dim.

This PR:

• Handles empty masks by returning [0, 0, 0, 0] bounding boxes
• Vectorizes the implementation (removes Python loop) for better performance on large batches
• Adds tests for empty and mixed empty/non-empty mask batches

This is based on the approach from #8194 but fixes the empty mask handling issue that was identified there (the vectorized argmax returns [0, 0, W-1, H-1] for empty masks instead of [0, 0, 0, 0]). We detect empty masks explicitly and set their boxes to zeros.

[pytorch-bot]

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