Add portable ELU implementation + test

[swolchok]

No description provided.

[swolchok]

Stack from ghstack (oldest at bottom):

• Add optimized ELU implementation #9521
• Bump PyTorch pin to 2025-03-25 #9578
• add sample input for new core op elu.out #9577
• -> Add portable ELU implementation + test #9520

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/9520

• 📄 Preview Python docs built from this PR

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❌ 2 New Failures, 1 Unrelated Failure

As of commit aafeb54 with merge base f13aeff ():

NEW FAILURES - The following jobs have failed:

• pull / unittest / macos / macos-job (gh)
  backends/xnnpack/test/passes/test_convert_to_linear.py::TestConvertToLinear::test_fp32_convert_to_linear
• pull / unittest-arm / linux-job (gh)
  RuntimeError: Command docker exec -t d2c4f3af436b294f22492288a9fb1ae98bd4591ad90c86cb6a3a733534f97e22 /exec failed with exit code 1

BROKEN TRUNK - The following job failed but were present on the merge base:

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

• pull / test-llava-runner-linux / linux-job (gh) (trunk failure)
  test_generated_output

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[kimishpatel]

did we actually decide to make it a portable op @larryliu0820? If we just want to control its decomp behavior we have other ways to do it as well, right?

[swolchok]

did we actually decide to make it a portable op @larryliu0820? If we just want to control its decomp behavior we have other ways to do it as well, right?

Do we not need portable op implementations for all core ATen ops?

[kimishpatel]

did we actually decide to make it a portable op @larryliu0820? If we just want to control its decomp behavior we have other ways to do it as well, right?

Do we not need portable op implementations for all core ATen ops?

Sorry I should have been more clear. I was actually asking "did we actually decide to it a core aten op?". If so yes

[swolchok]

core aten op

@kimishpatel pytorch/pytorch#149684