Hi, I have a question about the default RPNHead constructor, following is the source code from torchvision==0.6.1, located at models/detection/faster_rcnn.py around line 186:
if rpn_head is None:
    rpn_head = RPNHead(
        out_channels, rpn_anchor_generator.num_anchors_per_location()[0]
    )

My question is: Why the RPNHead is constructed with anchors as rpn_anchor_generator.num_anchors_per_location()[0] instead of sum(rpn_anchor_generator.num_anchors_per_location())?
For example: if I constructed rpn_anchor_generator with following code:
anchor_sizes = ((32,), (64,), (128,), (256,), (512,))
aspect_ratios = ((0.5, 1.0, 2.0),) * len(anchor_sizes)
rpn_anchor_generator = AnchorGenerator(
    anchor_sizes, aspect_ratios
)

I should get 15 anchors per location on the original Image? but with rpn_anchor_generator.num_anchors_per_location()[0] I could get only 3 anchors  per location.