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

models/yolo.py

@@ -110,8 +110,8 @@ class Model(nn.Module):
             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