[ExecuTorch][WebGPU] linear_q4gsw test suite: Llama-1B shapes + 4k/8k sweep

[JulianCloudNTH]

Stack from ghstack (oldest at bottom):

• [ExecuTorch][WebGPU] Add et_vk.prepack (constant-tensor packing) for E2E weight loading #20265
• [ExecuTorch][WebGPU] Add et_vk.apply_rotary_emb (interleaved RoPE) + ValueList multi-output #20264
• [ExecuTorch][WebGPU] Add et_vk.embedding_q4gsw (4-bit groupwise-symmetric quantized embedding) #20263
• -> [ExecuTorch][WebGPU] linear_q4gsw test suite: Llama-1B shapes + 4k/8k sweep #20227

Adds the numerical test suite for et_vk.linear_q4gsw (stacked on the op diff), mirroring the SDPA test suite. A named CONFIGS sweep covers real Llama-3.2-1B linear shapes — q/o-proj (2048->2048), k/v-proj (2048->512), gate/up-proj (2048->8192), down-proj (8192->2048), lm_head (2048->128256) — plus 4k/8k large-token prefill (M=4096/8192 on the 2048->2048 and 2048->512 projections). test/ops/quantized_linear/test_quantized_linear.py exports each config's .pte + an fp64 dequant-matmul "truth" golden; test/test_webgpu_native.cpp reconstructs the deterministic ramp input bit-for-bit, runs the op on the GPU, and compares per element; scripts/test_webgpu_native_ci.sh wires the fixtures into the Dawn(Tint)+SwiftShader CI.
@exported-using-ghexport

Differential Revision: D108314849

Differential Revision: D108314849

[pytorch-bot]

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[JulianCloudNTH]

@claude review

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Claude finished @JulianCloudNTH's task in 1m 32s —— View job

---

Code Review: PR #20227 — linear_q4gsw test suite

• Read PR diff
• Analyze test suite (Python export + C++ native test + CI wiring)
• Post review

Reviewed the four files. This is a clean, well-documented suite that closely mirrors the existing SDPA harness. The design is sound: a single CONFIGS table is the source of truth, the C++ kQ4gswConfigs table mirrors it, the deterministic ramp is reconstructed bit-for-bit, and the fp64 dequant-matmul "truth" golden is the right oracle. The required/heavy/self-skip + required && !dir.empty() ⇒ FAIL semantics correctly turn a silent export failure into a hard CI failure. No correctness bugs found.

A few observations, mostly minor:

1. Tolerance looser for 4k/8k variants — intentional? (worth confirming)

Per-element error in a dequant-matmul scales with the reduction dim K, not with M (number of token rows). Yet the large-M variants are 10× looser than their M=1 counterparts at the same K=2048:

{"q_proj",    1,    2048, 2048, 1e-4f, 1e-3f, ...},  // K=2048
{"q_proj_4k", 4096, 2048, 2048, 1e-3f, 1e-2f, ...},  // K=2048, 10x looser
{"q_proj_8k", 8192, 2048, 2048, 1e-3f, 1e-2f, ...},  // K=2048, 10x looser

down_proj's looser tol is well-justified (K=8192, big-K accumulation) and even has a // big-K accum comment. But for q_proj_4k/kv_proj_4k/q_proj_8k/kv_proj_8k the reduction dim is unchanged from the tight M=1 configs, so the looser bound just weakens the check unless the large-M kernel takes a different tiling/accumulation path that genuinely needs it. If it's the latter, a one-line comment (like the down_proj one) would make the intent explicit; if it's just conservatism, the M=1 tol (1e-4/1e-3) would be a stronger gate.

2. test_export_delegates comment slightly misleading for lm_head

# Fusion is shape-independent, so M=1 keeps even the heavy configs cheap to check.

For lm_head the weight is N×K/2 = 128256*2048/2 ≈ 131 MB regardless of M, so M=1 keeps the input/golden cheap but the exported .pte/in-memory packed weight is still large. The check is correct; the comment implies M=1 makes lm_head cheap overall, which isn't quite true. Minor wording nit. Fix this →

3. sdpa_within_tol is now a special case of quant_within_tol

quant_within_tol is the parameterized version (atol/rtol as args) of the hardcoded sdpa_within_tol (1e-4/1e-3). Not a problem for this PR, but sdpa_within_tol could later be replaced by quant_within_tol(..., 1e-4f, 1e-3f, ...) to drop the duplication. Optional cleanup.

Things I verified are correct

• Ramp bit-identity: ((i % 17) - 8) / 16 produces small integers over a power-of-two denominator — exactly representable in fp32, and the q4gsw_ramp(0) == -0.5f guard runs unconditionally. Python (np.int64 → % 17 → fp32 /16) matches C++ bit-for-bit. ✅
• Group-size constraints: group_size=32 satisfies K%32==0 (2048, 8192) and N%8==0 (2048, 512, 8192, 128256) for every config. ✅
• Heavy/required consistency: export_all_..._models(include_heavy=False) skips exactly the configs the C++ table marks heavy && !required, so CI gets clean self-skips (no FAIL) for lm_head/q_proj_8k/kv_proj_8k. ✅
• Path joining: Python os.path.join('/tmp', ...) and C++ dir + "q4gsw_..." with the trailing-slash normalization agree on /tmp/q4gsw_<name>.pte. ✅
• CI failure propagation: export || echo WARN + .pte absent + required + non-empty dir ⇒ native test FAILs, so a broken export can't pass silently. ✅
• int indexing: largest in_numel is q_proj_8k = 8192×2048 ≈ 16.8M, well within int range. ✅

Nothing blocking — items 1–3 are nits/clarifications. Item 1 is the only one worth an explicit answer.
· branch gh/JulianCloudNTH/24/head

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