[FEA] Support configurable indexing and NVE layers for exportable embedding collections

[geoffreyQiu]

Summary

Extend DynamicEmb exportable inference embedding collections with configurable indexing and NVE storage.

The conversion API now accepts an InferenceEmbeddingCollectionConfig for each physical embedding collection instead of selecting the implementation through use_dynamic.

What changed

• Add configurable FusedIdentityIndexer, LinearHashMapIndexer, and BitConcatIndexer.
• Support NVE GPU, LinearUVM, and hierarchical layers.
• Construct hierarchical tables directly through their parameter server using fused keys and embeddings.
• Keep data-backed indexer state and NVHashMap contents outside the exported PT2 package as sidecars.
• Support Redis-backed hierarchical-table incremental updates through the update coordinator and runtime subscriber.
• Add Python and C++ loading paths for exported indexer state.
• Add an end-to-end export, AOTI replay, sidecar loading, and incremental-load example.
• Update HSTU inference conversion and export call sites to pass the new collection configuration.

Verified combinations

Indexer	NVE layer/backend
FusedIdentityIndexer	GPU
LinearHashMapIndexer	LinearUVM
LinearHashMapIndexer	Hierarchical NVHashMap
LinearHashMapIndexer	Hierarchical Redis
BitConcatIndexer	Hierarchical NVHashMap
BitConcatIndexer	Hierarchical Redis

BitConcatIndexer is intentionally limited to hierarchical layers. Its table-ID and feature-ID bit widths are fixed by the model configuration.

Validation

• Passed the NVE 26.07 end-to-end GPU workflow.
• Verified all six embedding-collection combinations.
• Verified eager and Python AOTI execution.
• Verified C++ AOTI replay with sidecar loading.
• Verified Redis incremental loading through the coordinator/subscriber path.
• The example includes the NVE 26.05-compatible four-combination path; separate 26.05 GPU validation remains pending.

Commit organization

The draft is organized into three commits so it can later be split if needed:

1. Reusable DynamicEmb exportable-embedding implementation.
2. End-to-end DynamicEmb example and verification workflow.
3. HSTU integration with the new APIs.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.