Two-pass optimization for channelwise gated delta rule kernel (#21020)

[JakeStevens]

Summary:

Reduce the channelwise gated delta rule kernel from four state traversals to two per token. The first pass computes the predicted value while folding in decay; the second applies decay and the rank-one update while accumulating the output. This preserves operation grouping and numerical behavior while reducing state-memory traffic. Add a standalone microbenchmark covering decode and representative prefill lengths.

Differential Revision: D112596724

[pytorch-bot]

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❗ 2 Merge Blocking SEVs

There is 2 active merge blocking SEVs. Please view them below:

• (merge blocking) OOM on hf-cache causing jobs to fail at "Initialize Containers"
• (merge blocking) [Maintenance] - Due to #190347 we're advancing a maintenance planned to Sunday to today

If you must merge, use @pytorchbot merge -f.

❌ 3 New Failures

As of commit baee844 with merge base 931ba24 ():

NEW FAILURES - The following jobs have failed:

• periodic / test-models-linux (buck2, mv3, portable, linux.2xlarge, 90) / linux-job (gh)
  Process completed with exit code 1.
• pull / unittest-buck / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 242bc954699b5e3332eab5641a068ace189ae1ff8f1f5b05d4b694d2eb0f228e /exec failed with exit code 3
• pull / unittest-buck / macos / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 3

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[meta-codesync]

@JakeStevens has exported this pull request. If you are a Meta employee, you can view the originating Diff in D112596724.

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