Kill backend-specific lookup in CrossMapLRN2d, as it never succeeds.
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/25331
Test Plan: Imported from OSS
Differential Revision: D17097300
Pulled By: gchanan
fbshipit-source-id: 571e9691da13d34206ff3aabb8cb0cd1e82f6097
