layered config support dep

[jh-nv]

Summary

• Proposes layered YAML configuration support for Dynamo using OmegaConf for config composition
• Config files support single-parent inheritance, interpolation across the merged tree, and a flat
  root-level flag namespace
• Strict precedence model: CLI > env vars > config file > code defaults
• All config values flow through existing argparse/backend parser paths — no separate coercion layer

Key Design Decisions

• OmegaConf handles merge and interpolation only; parsers remain the source of truth for type
  conversion and validation
• Six-step pipeline: bootstrap config, collector parse for CLI provenance, build merged tree,
  resolve interpolation, compile to synthetic argv, parse through existing flow
• Covers vLLM, SGLang, and TRT-LLM backend integration
• Explicitly excludes: multiple inheritance, Hydra, custom resolvers, Rust-side loading