trunk/93e4cf28820c295e565916ab325b151c6065543c: Make AOTConfig immutable in AOT stage 2 (#184070)

Summary

Root cause:

• aot_stage2_compile() mutated AOTState.aot_config in place to inject stage-2 compilers and partitioners, and that made it hard to reason about whether aot_config represented the original compile request or a later stage-local override.
• Freezing AOTConfig also exposed a few other in-place updates (decompositions, cache_info) that needed to become whole-object replacements or local copies.

Proposed fix:

• Mark AOTConfig as frozen.
• Replace the remaining setup/cache-time config writes with whole-object replacement or stage-local copies.
• Thread the stage-2 partition_fn, fw_compiler, inference_compiler, and lazy-backward bw_compiler explicitly through the stage-2 helpers and runtime wrappers instead of stashing them on aot_config.
• Add regression coverage for both lazy backward and inference so stage 2 leaves the config untouched.

Why this is the right long term fix:

• It gives AOTConfig a single stable meaning again, keeps stage-specific compiler choices local to the code that consumes them, and preserves existing lazy-backward/inference behavior without hidden mutable state.

Drafted via Codex, published after manual review by @bobrenjc93

Pull Request resolved: #184070
Approved by: https://github.com/ezyang