Reland two-pass channelwise gated delta rule kernel + OSS-safe bench BUCK

[JakeStevens]

Summary:
Relands the two-pass optimization for the channelwise_gated_delta_rule custom op (originally #21020, D112596724), which was reverted in D113048961 because it broke OSS unittest macos / linux.

The revert was caused by the benchmark BUCK target:

runtime.python_binary(name = ..., srcs = [...], main_module = ...)

Fix: move the source into a runtime.python_library and have the runtime.python_binary reference it via deps with only main_module

Differential Revision: D113076546

[pytorch-bot]

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• Unrelated MacOS MPS test jobs time out (30 min) due to stale cpp_extension lock file poisoning self-hosted runners

❌ 2 New Failures

As of commit da0f64d with merge base 666ba0b ():

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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 D113076546.

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