Fuse consecutive clamps into a single clamp (#21013)

[apullin]

Summary:

ConvertToClampPass normalizes hardtanh, relu and relu6 into aten.clamp. Chains like HardTanh -> ReLU therefore become two adjacent clamps, the second of which is redundant.

Add FuseConsecutiveClampsPass, which folds any chain of adjacent (single-user) clamp.default ops into one clamp using exact composition: clamp(clamp(x,a,b),c,d) == clamp(x, max(a,c), min(b,d)). This subsumes HardTanh->ReLU, ReLU->ReLU, HardTanh->HardTanh, ReLU6->ReLU, explicit clamp->clamp, and longer chains, dropping one op (and one intermediate tensor) per fused pair.

Runs after FoldAndAnnotateQParamsPass so it fires in both the FP and INT paths; in the quantized case the surviving clamp inherits the first clamp's input qparams and the second's output qparams, dropping the intermediate requant (qtol=1, same tradeoff as FuseConsecutiveRescalesPass). Branching clamps (multiple users) are left untouched.

Differential Revision: D112351522

[pytorch-bot]

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❌ 1 New Failure, 1 Pending, 1 Unrelated Failure, 1 Unclassified Failure

As of commit 6da120a with merge base 0dec2b6 ():

NEW FAILURE - The following job has failed:

• Apple / build-frameworks-ios / macos-job (gh)
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• trunk / test-models-macos-cpu (resnet50, xnnpack-quantization-delegation) / macos-job (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 128

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

• pull / test-arm-backend-no-driver (test_pytest_models_tosa) / linux-job (gh) (trunk failure)
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[meta-codesync]

@apullin has exported this pull request. If you are a Meta employee, you can view the originating Diff in D112351522.

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[rascani]

Can you fix up the lintrunner? Thanks!

[apullin]

Should be good to go, now.