[Inductor] decompose expm1 for CPP vec (#92289)
For micro-bench op `aten.elu.default` in TIMM, the performance is not good even though with vectorization. `Elu` uses `expm1` as a sub-op. It turns out that inductor invokes sleef `expm1` function while aten decomposes it with `exp - 1`. The former one performs worse than the latter one. This PR decomposes `expm1` for cpp vectorization to make performance come back.
Performance data for eager v.s. inductor:
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suite | improved_ratio_speedup | speedup_old | RSD(3) | speedup_new | RSD(3)
-- | -- | -- | -- | -- | --
timm | 114.38% | 0.803447768 | 8.39% | 1.722458 | 27.74%
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Pull Request resolved: https://github.com/pytorch/pytorch/pull/92289
Approved by: https://github.com/jgong5, https://github.com/jansel
