[pythonic config][alternate] properly propagate default values set at the top level for nested config classes

[benpankow]

Summary

Alternate implementation of #27206 which relies on the partial config utilities from #12870

Ensures that default values on Pythonic Config get properly propagated to inner Config classes, e.g.

class ANestedOpConfig(Config):
    an_int: int = 1
    a_bool: bool = True

class AnOpConfig(Config):
    a_string: str = "foo"
    a_nested: ANestedOpConfig = ANestedOpConfig(an_int=5)

Test Plan

New unit test; todo add a few more

[benpankow]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• [pythonic resources] never treat partial resources as resolved #27210
• [pythonic config][alternate] properly propagate default values set at the top level for nested config classes #27209 👈 (View in Graphite)
• [pythonic resources] Partial config support for resources #12870
• [pythonic resources] Improve nested resource evaluation logic #14807
• master

This stack of pull requests is managed by Graphite. Learn more about stacking.