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)
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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]

I think you can uses functools partials for this, might be a bit more stable