fix centernet_head size_target (#4627)

ppdet/modeling/heads/centernet_head.py

@@ -201,9 +201,14 @@ class CenterNetHead(nn.Layer):
                 size_target = inputs['size']
                 # shape: [bs, max_per_img, 4]
             else:
-                size_target = inputs['size'][:, :, 0:2] + inputs['size'][:, :,
-                                                                         2:]
-                # shape: [bs, max_per_img, 2]
+                if inputs['size'].shape[-1] == 2:
+                    # inputs['size'] is wh, and regress as wh
+                    # shape: [bs, max_per_img, 2]
+                    size_target = inputs['size']
+                else:
+                    # inputs['size'] is ltrb, but regress as wh
+                    # shape: [bs, max_per_img, 4]
+                    size_target = inputs['size'][:, :, 0:2] + inputs['size'][:, :, 2:]
 
             size_target.stop_gradient = True
             size_loss = F.l1_loss(