Remove generic versions of aten ops and use portable instead

[mcremon-meta]

Summary: As titled, we don't need to maintain references for those ops since portable exists.

Differential Revision: D82667318

[pytorch-bot]

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❌ 1 New Failure

As of commit 1082137 with merge base 5348ea9 ():

NEW FAILURE - The following job has failed:

• pull / unittest-editable / macos / macos-job (gh)
  backends/xnnpack/test/ops/test_conv2d.py::TestConv2d::test_fp16_conv2d

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[facebook-github-bot]

@mcremon-meta has exported this pull request. If you are a Meta employee, you can view the originating diff in D82667318.

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