Add chunked prefill to the eagle3 runner for prompts past the ring-cache cap

[digantdesai]

A single target prefill forward caps at 2sliding_window (the sliding-ring KV
limit), so prompts beyond that must be chunked. Loop the prompt in window-sized
chunks for both the target prefill and the draft seed -- advancing input_pos,
accumulating the flat global KV cache, and concatenating the per-chunk features
-- then begin the speculative loop from the prompt end. Chunks are capped at the
sliding window (not 2window) for the same exactness reason as the gemma4 runner
fix: a larger chunk truncates sliding attention across chunk boundaries
(get_sliding_window from metadata, else max_prefill/2).

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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• [ROCm] MI350 CI runner label rename: rebase PRs using old linux.rocm.gpu.gfx950.* labels
• [Housekeeping] Rename uw1 cluster to meta-prod-aws-uw1

❌ You can merge normally! (1 Unrelated Failure), 2 Unclassified Failures

As of commit 0ab6f31 with merge base da9158b ():

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)
  /__w/executorch/executorch/pytorch/executorch/backends/apple/coreml/runtime/inmemoryfs/inmemory_filesystem.cpp:722:48: error: ‘inmemoryfs::InMemoryFileSystem::InMemoryNode::Kind’ has not been declared
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  Unable to download artifact(s): Artifact not found for name: pytorch_executorch__3.10_cpu_aarch64

BROKEN TRUNK - The following job failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / test-llama-runner-qnn-linux (fp32, qnn_16a16w, qnn) / linux-job (gh) (trunk failure)

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