[Fix] inconsistent dtype of seg_label in stdc decode

[MeowZheng]

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Motivation

fix #1462

Modification

• revise seg_label.float to seg_label.to(self.laplacian_kernel) in mmseg/models/decode_heads/stdc_head.py

BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repos?
If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.

Use cases (Optional)

If this PR introduces a new feature, it is better to list some use cases here, and update the documentation.

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMDet3D.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[codecov]

Codecov Report

Merging #1463 (4660f80) into master (3432ea9) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1463   +/-   ##
=======================================
  Coverage   90.31%   90.31%           
=======================================
  Files         139      139           
  Lines        8303     8304    +1     
  Branches     1395     1395           
=======================================
+ Hits         7499     7500    +1     
  Misses        567      567           
  Partials      237      237           

Flag	Coverage Δ
unittests	90.31% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
mmseg/models/decode_heads/stdc_head.py	100.00% <100.00%> (ø)
mmseg/datasets/pipelines/loading.py	98.55% <0.00%> (+0.02%)	⬆️

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