Issue #632: rename neg_weight to background_weight

[mathpluscode]

Description

Fixes #632

Type of change

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• [ ] Bugfix (non-breaking change which fixes an issue)
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• [ ] New feature (non-breaking change which adds functionality)
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Checklist

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[codecov[bot]]

Codecov Report

Merging #699 (c544ad6) into main (6a9a3ac) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #699   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           38        38           
  Lines         2423      2426    +3     
=========================================
+ Hits          2423      2426    +3     

Impacted Files	Coverage Δ
deepreg/config/v011.py	100.00% <100.00%> (ø)
deepreg/loss/label.py	100.00% <100.00%> (ø)

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[mathpluscode]

An arg renaming neg_weight to background_weight, backward compatibility is added with unit tests.

@YipengHu @kate-sann5100 @NMontanaBrown