Support extra_file arg

[cehongwang]

Description

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Fixes # (issue)

Type of change

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project (You can use the linters)
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas and hacks
• I have made corresponding changes to the documentation
• I have added tests to verify my fix or my feature
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[narendasan]

Just add a simple test to make sure the extra file is there after deserialization

[narendasan]

alternatively do you think its a good idea to just pass generic kwargs to the underlying save and load?

[narendasan]

@cehongwang can you retrigger tests?

[narendasan]

LGTM