And also, is it correct to get the corresponding feature location of every anchor in this way? It is in a ResNet50+FPN backbone Faster RCNN
anchor_sizes = ((32,), (64,), (128,), (256,), (512,)), 
aspect_ratios = ((0.5, 1.0, 2.0),) * len(anchor_sizes)
ag = AnchorGeneratorMP( anchor_sizes, aspect_ratios)
anchors, anchor_len = ag(image_list, features)
features = list(features.values())
anchor = anchors[:anchor_len[0]]
anchor = anchor.reshape(features[0].size()[-2:])
# is anchor[i][j] corresponding to features[0][:][:][i][j]?