Address failing inf/nan gelu test (#2046)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2046

ATen doesn't consistently handle the inf/-inf case. Hence, skipping test for ATen

Also discovered, while looking into this, that the portable kernel would return -nan when the input was -inf. However, gelu(-inf) should be 0.

Updated the portable kernel to ensure the proper handling of the infinities. Also updated the test, since the -inf case wasn't tested before.

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Reviewed By: kimishpatel, dbort

Differential Revision: D53960466

fbshipit-source-id: 8d19f06e2292b7ca7691705fd7e3dbbe8196a61e