GAN models added.

[ibrahimethemhamamci]

Fixes #ISSUE_NUMBER

Proposed Changes

Checklist

• [x] I have read the CONTRIBUTING guide
• [x] My PR is based from the current GaNDLF master
• [x] Non-breaking change (would not break existing functionality): please 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 submodule, add to list of exceptions for black styling in pyproject.toml file
• [ ] Usage documentation has been updated, if appropriate
• [ ] History has been updated, if appropriate
• [ ] Tests added or modified to cover the changes; if coverage is reduced, please give explanation

[sarthakpati]

Converting to draft until PR is reviewed.

[ibrahimethemhamamci]

Compute merging, codacy errors fixing, some unit tests

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