Added focal loss

[sarthakpati]

Fixes #695

Proposed Changes

• added focal loss and a hybrid focal loss with dice
• comment structure standardized on more files
• examples added

Checklist

• I have read the CONTRIBUTING guide.
• My PR is based from the current GaNDLF master .
• Non-breaking change (does not break existing functionality): provide as many details as possible for any breaking change.
• Function/class source code documentation added/updated.
• Code has been blacked for style consistency.
• If applicable, version information has been updated in GANDLF/version.py.
• If adding a git submodule, add to list of exceptions for black styling in pyproject.toml file.
• Usage documentation has been updated, if appropriate.
• Tests added or modified to cover the changes; if coverage is reduced, please give explanation.
• If customized dependency installation is required (i.e., a separate pip install step is needed for PR to be functional), please ensure it is reflected in all the files that control the CI, namely: python-test.yml, and all docker files [1,2,3].

[github-actions]

MLCommons CLA bot All contributors have signed the MLCommons CLA ✍️ ✅

[codecov]

Codecov Report

Merging #696 (cb6294a) into master (7fe43db) will increase coverage by 0.02%.
Report is 3 commits behind head on master.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #696      +/-   ##
==========================================
+ Coverage   94.67%   94.69%   +0.02%     
==========================================
  Files         117      117              
  Lines        8162     8200      +38     
==========================================
+ Hits         7727     7765      +38     
  Misses        435      435              

Flag	Coverage Δ
unittests	94.69% <100.00%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
GANDLF/compute/loss_and_metric.py	98.30% <100.00%> (ø)
GANDLF/losses/__init__.py	100.00% <100.00%> (ø)
GANDLF/losses/hybrid.py	100.00% <100.00%> (ø)
GANDLF/losses/segmentation.py	98.70% <100.00%> (+0.55%)	⬆️
GANDLF/parseConfig.py	84.78% <100.00%> (+0.25%)	⬆️
testing/test_full.py	99.16% <100.00%> (+<0.01%)	⬆️

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