[Feat][Linear-Attn] Update GDN prefill manifest contract

[superAngGao]

Summary

• update the GatedDeltaNetPrefillFwdOp spec-only manifest contract to allow chunk_size=None
• add the layout parameter used by the implementation path
• add an H16/S2K manifest workload row for the target prefill shape family

Notes

• This is a manifest-only precursor for PR [Perf][Linear-Attn] Add optimized Gated DeltaNet prefill op #1596.
• It intentionally keeps status: spec-only and leaves source paths null; the implementation PR should flip status/source/roofline once the kernel code lands.

Validation

• python scripts/validate_manifest.py --check-op GatedDeltaNetPrefillFwdOp

[gemini-code-assist]

Code Review

This pull request updates the GatedDeltaNetPrefillFwdOp manifest in tileops/manifest/linear_attention.yaml to support a nullable chunk_size (defaulting to null), introduces a new layout parameter defaulting to "bhtd", and adds a new benchmark workload. The review feedback suggests changing the default layout to "bhsd" to maintain consistency with the BHSD shape rules used elsewhere in the codebase, and adding a benchmark workload that explicitly tests the chunk_size: null configuration.

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