Added ability to perform stratified data splits

[scap3yvt]

Fixes #829

Proposed Changes

• added a new module under utils called data_splitter that allows a user to perform either stratified k-fold split or normal
• using the new module in training_manager

Checklist

• CONTRIBUTING guide has been followed.
• PR is based on 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 (ensure typing is used to provide type hints, including and not limited to using Optional if a variable has a pre-defined value).
• Code has been blacked for style consistency and linting.
• 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

Attention: Patch coverage is 96.47887% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 95.09%. Comparing base (0be31c2) to head (b003c3c).
Report is 1 commits behind head on master.

❗ Current head b003c3c differs from pull request most recent head 1d2352b. Consider uploading reports for the commit 1d2352b to get more accurate results

Files	Patch %	Lines
GANDLF/training_manager.py	84.84%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #831      +/-   ##
==========================================
+ Coverage   95.01%   95.09%   +0.07%     
==========================================
  Files         120      121       +1     
  Lines        8270     8312      +42     
==========================================
+ Hits         7858     7904      +46     
+ Misses        412      408       -4     

Flag	Coverage Δ
unittests	95.09% <96.47%> (+0.07%)	⬆️

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

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[sarthakpati]

Add another test, please.

[Geeks-Sid]

@scap3yvt tag me when it is ready to review.

[sarthakpati]

@scap3yvt tag me when it is ready to review.

This should be ready for review, @Geeks-Sid