[Fix] Fix multi-task-head loss potential bug

[Ezra-Yu]

Motivation

in the pull request #1229 , for the extreme case that we have a full-mask we do for the loss = 0 which will cause problems to calculate the gradient of the loss. here i use the torch vector of the image to create a loss =0

#1393

the same as #1426

Modification

Please briefly describe what modification is made in this PR.

Checklist

Before PR:

• Pre-commit or other linting tools are used to fix the potential lint issues.
• Bug fixes are fully covered by unit tests, the case that causes the bug should be added in the unit tests.
• The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, like docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with those projects, like MMDet or MMSeg.
• CLA has been signed and all committers have signed the CLA in this PR.

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +0.54 🎉

Comparison is base (c9a0cb0) 84.37% compared to head (af478ae) 84.91%.

❗ Current head af478ae differs from pull request most recent head fe72c37. Consider uploading reports for the commit fe72c37 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #1530      +/-   ##
==========================================
+ Coverage   84.37%   84.91%   +0.54%     
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  Files         142      230      +88     
  Lines        9925    17556    +7631     
  Branches     1621     2758    +1137     
==========================================
+ Hits         8374    14908    +6534     
- Misses       1277     2130     +853     
- Partials      274      518     +244     

Flag	Coverage Δ
unittests	84.91% <ø> (+0.54%)	⬆️

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