RFC-0057: Semantic Identity for Inductor Distributed Collectives

[NmomoN]

Summary

This RFC proposes a small, private semantic identity contract for functional distributed collectives in Inductor.

Inductor can identify collective transport types such as all-gather and reduce-scatter, but currently relies on graph structure to infer why those collectives were emitted. This becomes ambiguous when FSDP, tensor parallelism, context parallelism, and user-authored distributed code coexist in the same graph.

Phase 1 starts with FSDP. It introduces purpose-oriented collective roles, distinguishes producer-provided identity from heuristic inference, and defines how that identity should be preserved or invalidated by Inductor graph transformations.

This PR contains a design proposal only and does not change PyTorch behavior.

Scope

Phase 1 covers:

• private semantic roles for FSDP collectives;
• metadata-first classification with existing structural heuristics as fallback;
• validation of a private PT2 metadata transport;
• preservation and invalidation rules across graph transformations;
• diagnostics for missing, conflicting, or lost metadata.

It intentionally does not introduce scheduling priority, static urgency, topology-aware cost models, or a public annotation API.

Feedback requested

Feedback would be particularly helpful on:

1. whether FSDP is an appropriate first validation scope;
2. the proposed private metadata transport;
3. the FSDP role vocabulary and producer locations;
4. precedence between explicit metadata and structural inference;
5. metadata behavior across bucketing, deduplication, and fusion.

Related PyTorch issue: pytorch/pytorch#190792

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