Added functionality related to DP training

[scap3yvt]

Fixes #944

Proposed Changes

• Add DP-enabled training and inference from private repo
• Most of the code is copy-paste from the private repo, with some added documentation and changes from raised errors to assertions.

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].
• The logging library is being used and no print statements are left.

[github-actions]

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[sarthakpati]

This seems to be okay for me. Any comments, @VukW and @brandon-edwards?

[brandon-edwards]

This looks reasonable to me as well, but I have not tested it.

[sarthakpati]

This should not be merged until #947 is merged and the master gets tagged.

[sarthakpati]

Support ticket generated with Codacy to explore the coverage issue.