[cherry-pick][MOT][MOT] fix fairmot centernet_head bias init (#4207)

* fix operators typo

* fix centernet_head bias init

ppdet/modeling/heads/centernet_head.py

@@ -98,6 +98,7 @@ class CenterNetHead(nn.Layer):
                 stride=1,
                 padding=0,
                 bias=True))
+        with paddle.no_grad():
             self.heatmap[2].conv.bias[:] = -2.19
         self.size = nn.Sequential(
             ConvLayer(