fix: forbid extras in Configs

[benieric]

Issue #, if available:
Addresses: #5041

Description of changes:

• Fix bug where a cryptic UnboundLocalError is raised if an invalid SourceCode object is passed into ModelTrainer
• Details in issue
• Additionally addresses a small bug where destructor fails if called before class is fully initialized

AttributeError: 'ModelTrainer' object has no attribute '__pydantic_private__'

Testing done:

• Added unit test to make sure user cannot create an invalid SourceCode

Merge Checklist

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Tests

• I have added tests that prove my fix is effective or that my feature works (if appropriate)
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[mufaddal-rohawala]

Looks like from if else ladder above, "please provide a valid SourceCode or distributed or base_command?"

[benieric]

Yeah, thats true let me make this clearer to prevent confusion