Clean up bisect_percentile_op (#73148)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/73148
Makes a bunch of things const, eliminates extraneous variables
Test Plan: Sandcastle
Reviewed By: malfet
Differential Revision: D34365183
fbshipit-source-id: 56e4c43e0c14d28f9d18903e9b05f993637489b1
(cherry picked from commit 51520edd16084270aefe8f8143799f918d7ae22d)
