[FEATURE] Add slimmable size breakpoint introspection

[sdatkinson]

Description

Add SlimmableModel::GetSlimmableSizeBreakpoints() so hosts can enumerate the normalized size-control values where a slimmable model changes size.

Resolves #268.

Summary:

• Add breakpoint introspection to the shared SlimmableModel interface.
• Implement breakpoints for SlimmableContainer.
• Implement breakpoints for SlimmableWavenet, using the sorted unique union of internal ratio dividers across slimmable layer arrays.
• Use internal divider semantics: 0.0 and 1.0 are implied bounds, so n selectable sizes return n - 1 breakpoints.
• Add tests for container and WaveNet breakpoint behavior.
• Leave a TODO to make the new interface method abstract in the next breaking release.