Fold DyT alpha scalar into tanh LUT

[cgreenberg]

Summary:
Dynamic Tanh (DyT) normalization computes tanh(alpha * x) with a learned scalar
alpha. Quantized, that lowers to a full-tensor integer Mul followed by a tanh
TABLE. On Ethos-U the TABLE is free but the Mul is a real per-element cost, so
the Mul is pure overhead.

This adds FoldDyTAlphaIntoLUTPass in backends/arm/_passes/, which folds the
alpha multiply into the tanh lookup table and deletes the Mul and its
surrounding rescales.

Folding alpha in floating point before quantization would change rounding, so the
pass does it in the integer domain instead: it replays the exact TOSA
SINGLE_ROUND RESCALE and Mul arithmetic over all 256 int8 input codes, feeds the
results through the existing tanh quantization mapping, and materializes the
result as one 256-entry TABLE. The rewrite is therefore byte-exact rather than
approximate. It fails closed on anything it cannot prove: non-scalar alpha,
activation-side rank views, and narrowed tanh ranges are all handled explicitly.

Also adds register_pass_factories_before() to the Arm pass manager, a small
hook for inserting a pass that needs access to the ExportedProgram ahead of a
named target pass. FoldDyTAlphaIntoLUTPass needs it to read constant tensors.

Note on one import: the pass imports register_pass_factories_before inside
register_fold_dyt_alpha_into_lut_pass() rather than at module scope.
_passes/__init__.py imports arm_pass_manager last and arm_pass_manager
imports back from the package, so a module-scope import from a pass module that
__init__.py re-exports is circular. Only the register helper needs it.

The pass is inert until a model registers it, so this diff changes no behaviour
on its own.

Differential Revision: D116560649

cc @digantdesai @freddan80 @per @zingo @oscarandersson8218 @mansnils @Sebastian-Larsson @robell @rascani

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❌ 2 New Failures, 3 Unrelated Failures

As of commit 54b33a0 with merge base 1b2838b ():

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FLAKY - The following jobs failed but were likely due to flakiness present on trunk:

• pull / test-models-linux (linear, portable, linux.2xlarge) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.
• pull / test-vulkan-operators-linux / linux-job (gh) (matched linux rule in flaky-rules.json)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.

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

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

• Apple / build-demo-ios / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 65

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@cgreenberg has exported this pull request. If you are a Meta employee, you can view the originating Diff in D116560649.

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[JakeStevens]

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