fix: exact 320x320 resize for fixed-shape u2netp

preprocessor_config.json

@@ -17,7 +17,8 @@
   "resample": 2,
   "rescale_factor": 0.00392156862745098,
   "size": {
-    "shortest_edge": 320
+    "height": 320,
+    "width": 320
   },
   "size_divisibility": 32
 }