How can anchor box and NMS be applied to a test set when the Ground Truth is unknown?
Without the Ground Truth, the IOU of the anchor box is unknown, so are all the anchor boxes used to predict the box location?
Then, why are they used in training the first place.
1 - You don't need GT in the validation or test phase except measuring the performance of model. Because the model have already learned the regression coefficients in the training phase. These coefficients are used to fit the default anchor boxes to predictions.
2 - NMS eliminates the region proposals without using GT. It aims to select best proposal over the other proposals while comparing them against each other.
3 - If you don't know the GT you can't learn the classes and coordinates of objects during the training phase. So you need compare your GT with predictions. You can check out this great article to have comprehensive understanding of R-CNN family.