Fix dtype selective build check in elementwise_util

[swolchok]

I think I got this wrong in #11975 -- the compute_dtype is the one type that is NOT related to selective build.

[swolchok]

Stack from ghstack (oldest at bottom):

• -> Fix dtype selective build check in elementwise_util #12183

[pytorch-bot]

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❌ 1 New Failure, 1 Cancelled Job, 1 Unrelated Failure

As of commit 71d32cc with merge base 02454eb ():

NEW FAILURE - The following job has failed:

• pull / unittest-editable / linux / linux-job (gh)
  backends/xnnpack/test/ops/test_gelu.py::TestGelu::test_fp16_gelu

CANCELLED JOB - The following job was cancelled. Please retry:

• pull / test-models-linux (mobilebert, xnnpack-quantization-delegation, linux.2xlarge) / linux-job (gh)
  ##[error]The operation was canceled.

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-models-linux (mobilebert, portable, linux.2xlarge) / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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[swolchok]

• Kimish and Lucy because we just discussed this in person