Migrate xlnet to the semantic Task SDK #1537

Parent epic: #1430

Status: Not started — comment here before beginning work.

Family: families/xlnet/
Current declared task mode(s): embedding, encoding, reranking
Target semantic Task(s): Identify the existing semantic Task contract(s) that preserve every current capability; record the exact IDs in the PR.

Work

• Inventory this family's primary and secondary behavior, including batch, streaming, raw outputs and stateful operations where supported. Record the current defaults, profiles, checkpoint cases, and reference criteria.
• Implement and bind the matching semantic Task interface(s) in this family's runtime. Preserve its graph, preprocessing, inference, postprocessing, configuration and complete owned results.
• Align family-owned support, build metadata and manifests with the primary Task. Advertise only interfaces implemented by the loaded bundle; document whether existing bundles need rebuilding.
• Add family-owned CPU contracts and public C/C++ SDK consumers. Cover invalid inputs, configuration handling, result lifetime and every relevant lifecycle.
• Run a selected real checkpoint E2E with the existing oracle and thresholds, then check the remaining declared capabilities and public CLI, example and benchmark paths. Preserve meaningful passing criteria.

PR evidence

Link the migration PR here. Report the tested revision, exact commands and results, selected checkpoint cases, original reference checks, current-head CI, and any untested configurations. Keep a normal migration within families/xlnet/**; raise a separate linked prerequisite issue if a shared contract is genuinely missing. Do not weaken an acceptance criterion to pass CI.

References: #1226 · family validation guide.