Fix check_anchor_order() in pixel-space not grid-space (ultralytics#7060)

* Update `check_anchor_order()`

Use mean area per output layer for added stability.

* Check in pixel-space not grid-space fix

Author: glenn-jocher

models/yolo.py

@@ -110,8 +110,8 @@ def __init__(self, cfg='yolov5s.yaml', ch=3, nc=None, anchors=None):  # model, i
             s = 256  # 2x min stride
             m.inplace = self.inplace
             m.stride = torch.tensor([s / x.shape[-2] for x in self.forward(torch.zeros(1, ch, s, s))])  # forward
+            check_anchor_order(m)  # must be in pixel-space (not grid-space)
             m.anchors /= m.stride.view(-1, 1, 1)
-            check_anchor_order(m)
             self.stride = m.stride
             self._initialize_biases()  # only run once
 

utils/autoanchor.py

@@ -17,7 +17,7 @@
 
 def check_anchor_order(m):
     # Check anchor order against stride order for YOLOv5 Detect() module m, and correct if necessary
-    a = m.anchors.prod(-1).view(-1)  # anchor area
+    a = m.anchors.prod(-1).mean(-1).view(-1)  # mean anchor area per output layer
     da = a[-1] - a[0]  # delta a
     ds = m.stride[-1] - m.stride[0]  # delta s
     if da.sign() != ds.sign():  # same order