Add mask support to cadence softmax (#21596)

[DrJessop]

Summary:

The kernel supports masking, need to support it in the reference as well.

Reviewed By: aliafzal

Differential Revision: D114671838

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/21596

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❌ 2 Cancelled Jobs, 1 Unrelated Failure

As of commit 5a89d6f with merge base bb6b99a ():

CANCELLED JOBS - The following jobs were cancelled. Please retry:

• pull / unittest / macos / macos-job (gh)
• pull / unittest-editable / macos / macos-job (gh)

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / unittest-buck / macos / macos-job (gh) (detected as infra flaky with no log or failing log classifier)

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[meta-codesync]

@DrJessop has exported this pull request. If you are a Meta employee, you can view the originating Diff in D114671838.

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