Describe the bug
Anchors should be placed ideally in the middle of a grid cell.
However, this is not the case here:

Expected behavior
The anchors should be placed ideally in the middle of a grid cell.

Possible fix
This could probably be fixed by simply changing these lines to:

shifts_centers = [
    torch.arange(0, size[axis], dtype=torch.int32, device=device) * stride[axis] + int(stride[axis] / 2)
    for axis in range(self.spatial_dims)
]