Transforms: share the horizontal fusion passes

[rascani]

FuseDuplicateUsersPass and FuseIdenticalInputTransformsPass are the fan-out and
fan-in halves of horizontal fusion: the first merges identical transforms that
several users apply to one producer, the second merges identical transforms
several producers apply into one consumer. Together with the canonicalizer they
are everything the propagation driver needs besides the algorithm itself.

Neither had a real tie to Arm. FuseIdenticalInputTransformsPass took
ArmOpTargetedPass for a pre-scan optimization it never used, since it overrides
call() outright and reads target_ops only as a membership set.

FuseDuplicateUsersPass had one genuine dependency worth preserving rather than
deleting: it refuses to fuse duplicate users of tosa.RESCALE, because a fused
RESCALE feeding every original consumer corrupts integer output on Ethos-U. That
is not a property of duplicate-user fusion, it is a property of RESCALE, so the
exclusion set becomes a constructor argument that defaults to empty. The Arm
subclass supplies the TOSA set, and the driver asks for it through a new
duplicate_user_fusion_exclusions() extension point so it no longer needs to know
what a RESCALE is.

With this the driver's only remaining Arm import is the pass base class, and it
can move next.

No behaviour change: the Arm pass suite produces a failure set identical to
merge base, name for name.

Authored with Claude Code.

[rascani]

Stack from ghstack (oldest at bottom):

• Transforms: let a backend declare what counts as a permute #22125
• Transforms: stop fork splitting from stranding a permute #22124
• Transforms: share the permute/view propagation driver #22123
• -> Transforms: share the horizontal fusion passes #22122
• Transforms: share the view/permute canonicalizer #22121
• Arm backend: give the propagation pass backend extension points #22120
• Transforms: share the view/permute algebra with all backends #22119

[pytorch-bot]

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❌ 11 New Failures

As of commit 5f12829 with merge base e831b3e ():

NEW FAILURES - The following jobs have failed:

• Lint / lintrunner (gh)
  >>> Lint for backends/arm/_passes/propagate_view_copy_permute_pass.py:
• Lint / lintrunner-mypy (gh)
  >>> Lint for backends/arm/_passes/propagate_view_copy_permute_pass.py:
• pull / test-arm-backend-no-driver (test_pytest_models_tosa) / linux-job (gh)
  RuntimeError: Command docker exec -t 1459e24e734a2963b3f7f979adbd9ec50cf3c205c613f847f8270281a8f94b5e /exec failed with exit code 1
• pull / test-arm-backend-no-driver (test_pytest_ops_tosa) / linux-job (gh)
  RuntimeError: Command docker exec -t 1b4dfe9c342a6c0d2a54df3ae046b133d1de378e0983cf5f2a9cefdcf72fc7a1 /exec failed with exit code 1
• pull / unittest-buck / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 24cdd2dfeb41d305e98d1cca16fc25cb79aca4ab7bd4d2b33a89f8a05671321e /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
• trunk / test-arm-backend-ethos-u (test_model_smollm2_135M_ethos_u85) / linux-job (gh)
  RuntimeError: Command docker exec -t 17402815cfc1c042e6f91e2ee83f17731d806823d092d310b2f37864b3a4cc7a /exec failed with exit code 1
• trunk / test-arm-backend-ethos-u (test_pytest_ops_ethos_u55) / linux-job (gh)
  RuntimeError: Command docker exec -t e8f9bd5e8b4daca49ff51bb67b5869a7e7b7b3aa16cdf28eee94ec29592579c6 /exec failed with exit code 1
• trunk / test-arm-backend-ethos-u (test_pytest_ops_ethos_u85) / linux-job (gh)
  RuntimeError: Command docker exec -t 22b23b535416ab969663b62136bbf59be89af9c49b558876b02e3ad8fed1e315 /exec failed with exit code 1
• trunk / test-arm-backend-vkml (test_pytest_models_vkml) / linux-job (gh)
  RuntimeError: Command docker exec -t 88e18c0f2490f0b0e49c4b3da41cdcb15c574952dbc05d2b58b20c152ca5765e /exec failed with exit code 1
• trunk / test-arm-backend-vkml (test_pytest_ops_vkml) / linux-job (gh)
  RuntimeError: Command docker exec -t 7d48c150acee2df156c6e4f716719bdf3b233e8669574613a733b6eb24b4a003 /exec failed with exit code 1

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