Make EAGLE-3 chain length K a runtime parameter

[digantdesai]

target_verify was exported at a static T = chain+1, so K was baked into the .pte
and changing it meant a full re-export. Export the verify window as a dynamic dim
T in [2, MATVEC_MAX_M] instead, so one .pte serves any K in [1, MATVEC_MAX_M - 1]
and the runner selects it with --chain (get_chain_len is only the default). The
verify M never straddles the INT4 dispatch threshold (max == MATVEC_MAX_M), so it
resolves to the small-M GEMM over the whole range, and the mid-M SDPA kernel
takes M as a runtime arg. The gemma4 mask traces correctly down to T=2; the
target's min_forward_len=5 was a conservative prefill note and does not bound
verify.

Authored with assistance from Claude Code.

[digantdesai]

Stack from ghstack (oldest at bottom):

• Update tokenizers submodule to the linear-time HF encode path #20349
• Add --prompt_file flag to the eagle3 runner #20348
• Add chunked prefill to the eagle3 runner for prompts past the ring-cache cap #20347
• Fix sliding-window chunked prefill in the gemma4-31B runner #20346
• -> Make EAGLE-3 chain length K a runtime parameter #20345
• Add split-K mid-M SDPA Triton kernel for EAGLE-3 target_verify at long context #20344
• Add EAGLE-3 end-to-end speculative-decode test #20157
• Add the EAGLE-3 speculative-decoding runner (CUDA) #20156
• Add the EAGLE-3 speculator CUDA export #20155
• Add a weight-stationary INT4 GEMM for small M #20154
• Add the Eagle3Speculator module for speculative decoding #20153
• Add KV cache to the EAGLE-3 draft head #20152
• Add Gemma4 EAGLE-3 eager reference #20151
• Add Gemma4 EAGLE-3 hidden-state taps #20150
• Add EAGLE-3 draft head #20149

[pytorch-bot]

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❌ 5 New Failures, 1 Cancelled Job, 2 Unrelated Failures, 5 Unclassified Failures

As of commit 42ee17b with merge base dc55469 ():

NEW FAILURES - The following jobs have failed:

• pull / test-arm-backend-no-driver (test_pytest_models_tosa) / linux-job (gh)
  RuntimeError: Command docker exec -t 4f6e24d42f2b0435043ae1e9773324b76014a2b0e7cba3934d0df4163658ebe8 /exec failed with exit code 1
• pull / test-arm-backend-no-driver (test_pytest_ops_tosa) / linux-job (gh)
  RuntimeError: Command docker exec -t c801bb6ff234370acdfc91e6ea087b7d82584410013866d468c5303859ae1e8a /exec failed with exit code 1
• pull / test-lora-multimethod-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 06e3c08c03bb9f75fa20c05d66726da37b55a45558b04fd29f7d8b453151500a /exec failed with exit code 1
• pull / test-mcu-cortex-m-backend / linux-job (gh)
  RuntimeError: Command docker exec -t a59e386596e9256481ffe53ff489e41a4e6bd476acbbe9e9e8b875aee4d82515 /exec failed with exit code 1
• Test CUDA Windows Export and E2E / test-model-cuda-windows-e2e (facebook, dinov2-small-imagenet1k-1-layer, non-quantized) / windows-job (gh)
  Process completed with exit code 1.

UNCLASSIFIED FAILURES - DrCI could not classify the following jobs because the workflow did not run on the merge base. The failures may be pre-existing on trunk or introduced by this PR:

• Build Aarch64 Linux Wheels / pytorch/executorch / build-wheel-py3_10-cpu-aarch64 (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
• Build Aarch64 Linux Wheels / pytorch/executorch / upload / upload-wheel-py3_10-cpu-aarch64 (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  Unable to download artifact(s): Artifact not found for name: pytorch_executorch__3.10_cpu_aarch64
• Test CUDA Builds / export-model-cuda-artifact (SocialLocalMobile, gemma-4-31B-it-HQQ-INT4, quantized-int4-tile-packed) / linux-job (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
• Test CUDA Builds / export-model-cuda-artifact (SocialLocalMobile, Qwen3.5-35B-A3B-HQQ-INT4, quantized-int4-tile-packed) / linux-job (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
• Test CUDA Builds / unittest-cuda / linux-job (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  backends/cuda/tests/test_int4_dispatch.py::TestLimitConsistency::test_shim_gemm_max_m_matches_cuh

CANCELLED JOB - The following job was cancelled. Please retry:

• pull / unittest-nxp-neutron / linux-job (gh)
  ##[error]The operation was canceled.

FLAKY - The following jobs failed but were likely due to flakiness present on trunk:

• pull / test-moshi-linux / linux-job (gh) (matched linux rule in flaky-rules.json)
  E: Failed to fetch http://archive.ubuntu.com/ubuntu/pool/main/m/mesa/mesa-vdpau-drivers_23.2.1-1ubuntu3.1%7e22.04.3_amd64.deb 404 Not Found [IP: 104.20.28.246 80]
• pull / test-qnn-testsuite-linux / package-golden-artifacts (gh) (matched linux rule in flaky-rules.json)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.

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