trunk/804cbec994d64dc23db16429f0d2b83d5dacd3c8: Cache triton_key via torch_key_cache for prefetch support (#182188)

Wrap triton_key in a cached function decorated with torch_key_cache.

This enables prefetching the triton key in a background thread via

triton_key.prefetch(), allowing it to be overlapped with other startup

work like torch_key computation.

Also moves torch_key_cache and _MISSING earlier in the file so they

can be used by triton_key before CacheBase.

Authored with Claude.

Pull Request resolved: #182188

Approved by: https://github.com/zou3519

ghstack dependencies: #182187