Back out "Use simd version for fp16 conversions" (#32640)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/32640
Original commit changeset: 3b1ee0ba756e
Reverting according to https://our.intern.facebook.com/intern/diff/D19291499/?transaction_id=1347995678706116&dest_fbid=465672071047258
Test Plan: unittests.
Reviewed By: jspark1105, jianyuh
Differential Revision: D19576708
fbshipit-source-id: bec92318523498067935234ab702c925ece71da6
