671 customize layer channels in unet

[mathpluscode]

Description

Enable changing encoder/decoder channels in U-Net

Fixes #671

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)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Other (if none of the other choices apply)

Checklist

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• [x] 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.
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• [x] I have updated the change log file regarding my changes.
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[codecov[bot]]

Codecov Report

Merging #673 (51db404) into main (077bc4a) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #673   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           38        38           
  Lines         2412      2424   +12     
=========================================
+ Hits          2412      2424   +12     

Impacted Files	Coverage Δ
deepreg/model/backbone/u_net.py	100.00% <100.00%> (ø)

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[mathpluscode]

@s-sd @NMontanaBrown @YipengHu would you have time to review this :) ?