Issue #690: fix negative variance issue and add accurate test

[mathpluscode]

Description

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context. List any dependencies that are required for this change.

Fixes #690

Type of change

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• [ ] Bugfix (non-breaking change which fixes an issue)
• [ ] Code style update (formatting, renaming)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Documentation Update (fix or improvement on the documentation)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Other (if none of the other choices apply)

Checklist

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• [ ] I have installed pre-commit using pre-commit install and formatted all changed files. If you are not certain, run pre-commit run --all-files.
• [ ] My commits' message styles matches our requested structure, e.g. Issue #<issue number>: detailed message.
• [ ] I have updated the change log file regarding my changes.
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added necessary documentation (if appropriate)

[codecov[bot]]

Codecov Report

Merging #733 (531b681) into main (2cddc38) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #733   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           38        39    +1     
  Lines         2445      2455   +10     
=========================================
+ Hits          2445      2455   +10     

Impacted Files	Coverage Δ
deepreg/loss/util.py	100.00% <ø> (ø)
deepreg/model/network.py	100.00% <ø> (ø)
deepreg/constant.py	100.00% <100.00%> (ø)
deepreg/dataset/loader/interface.py	100.00% <100.00%> (ø)
deepreg/loss/image.py	100.00% <100.00%> (ø)
deepreg/loss/label.py	100.00% <100.00%> (ø)

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