Issue #613: remove loss weight check in sanity check

[mathpluscode]

Description

As now, _build_loss handles the checking and is quite robust. No more need to check losses in sanity checks.

https://github.com/DeepRegNet/DeepReg/blob/main/deepreg/model/network.py#L174

Fixes #613

Type of change

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[mathpluscode]

@YipengHu sorry for the delay. This fix was quite simple in fact.

I've tested the config mentioned in #613, it now works with a warning as expected.

Further changes linked to sanity checks will be addressed in https://github.com/DeepRegNet/DeepReg/issues/652

[codecov[bot]]

Codecov Report

Merging #688 (2ca3a81) into main (183d7f8) will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##              main      #688   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           38        38           
  Lines         2432      2424    -8     
=========================================
- Hits          2432      2424    -8     

Impacted Files	Coverage Δ
deepreg/config/parser.py	100.00% <ø> (ø)
deepreg/model/network.py	100.00% <ø> (ø)

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