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

* Update `check_anchor_order()`

Use mean area per output layer for added stability.

* Check in pixel-space not grid-space fix

utils/autoanchor.py

@@ -17,7 +17,7 @@ PREFIX = colorstr('AutoAnchor: ')
 
 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