Add EAGLE-3 end-to-end speculative-decode test

[digantdesai]

Tiny matching target + draft on CPU drive the shifted method-flow the runner
implements -- prefill, draft chain, target_verify, accept, reseed -- and the
generated tokens are checked against greedy decoding (lossless by construction).
Forced-acceptance cases pin the partial, full, and accepted-EOS paths plus the
one-token budget; the random-weight loop alone can leave them uncovered.

This covers the decoding algorithm on CPU only; the CUDA/AOTI export (export.py)
and the C++ runner are exercised manually. Registered in pytest.ini next to the
draft/speculator unit tests.

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

As of commit dc6a8e0 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:

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  /__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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