Object detection fails silently when using Detectron2 with multiprocessing and CUDA 🐛 Bugs

Object detection fails silently when using Detectron2 with multiprocessing and CUDA — the model loads correctly, but no objects are detected unless run in the main process or on CPU.

Start by reproducing object detection with Detectron2 under multiprocessing with CUDA, then compare it with the main-process and CPU cases described in the issue. Investigate why the model loads but produces no detections; done means detection works reliably in the multiprocessing CUDA scenario and the regression is covered by an appropriate test or reproduction.

Written by the indexing model from the issue text.