[Design][Linear-Attn] Complete GDN prefill manifest contract

[superAngGao]

Summary

• encode the GatedDeltaNetPrefillFwdOp BHTD/BHSD and BTHD layout shape contracts in the manifest
• add fixed-rank shape metadata for the manifest tensors
• fill the planned roofline/source metadata while keeping the entry spec-only

Notes

• This is a manifest-only prerequisite for PR [Perf][Linear-Attn] Add optimized Gated DeltaNet prefill op #1596.
• The implementation PR should only flip status and keep any remaining manifest edit within the status-flip carve-out.

Validation

• python scripts/validate_manifest.py

[gemini-code-assist]

Code Review

This pull request updates the GatedDeltaNetPrefillFwdOp manifest in linear_attention.yaml by defining input/output shapes, adding layout-dependent shape rules, and specifying source paths and a roofline function. However, the reviewer noted that referencing non-existent source files, classes, and the roofline function in a spec-only manifest can break validation and test runners. It is recommended to revert these fields to placeholders or null until the actual implementation is introduced.

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