Add Gemma4 EAGLE-3 eager reference

[digantdesai]

Adds a CUDA eager speculative-decoding reference that loads the
gemma4-31B target and EAGLE-3 draft head, runs greedy-chain
verification, clips EOS and generation-budget emissions, and reports
acceptance metrics.

Authored with assistance from Claude Code.

[digantdesai]

Stack from ghstack (oldest at bottom):

• 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]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/20151

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⏳ No Failures, 4 Pending

As of commit 11d8d07 with merge base dc55469 ():
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