[Fix] inconsistent dtype ofseg_label in stdc decode (open-mmlab#1463)

Author: MeowZheng

mmseg/models/decode_heads/stdc_head.py

@@ -37,7 +37,7 @@ def losses(self, seg_logit, seg_label):
         # parameters. However, it is a constant in original repo and other
         # codebase because it would not be added into computation graph
         # after threshold operation.
-        seg_label = seg_label.float()
+        seg_label = seg_label.to(self.laplacian_kernel)
         boundary_targets = F.conv2d(
             seg_label, self.laplacian_kernel, padding=1)
         boundary_targets = boundary_targets.clamp(min=0)