Add non-zero da check_anchor_order() condition (ultralytics#7066)

Author: glenn-jocher

utils/autoanchor.py

@@ -20,7 +20,7 @@ def check_anchor_order(m):
     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
+    if da and (da.sign() != ds.sign()):  # same order
         LOGGER.info(f'{PREFIX}Reversing anchor order')
         m.anchors[:] = m.anchors.flip(0)