Use compile-time promotion for op_add size/build time

[swolchok]

Summary: Yet another smaller operator.

Differential Revision: D56742276

[pytorch-bot[bot]]

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:test_tube: See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/3418

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As of commit ae77223de147c85923e33d87bb62245c1d7b58f5 with merge base 34aadcbb969a2e23c672b89a5fba19bc937eaf83 (): :green_heart: Looks good so far! There are no failures yet. :green_heart:

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[swolchok]

this landed