Make LazyIr.h use provided backend namespace (#75264)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/75264
Fixes one of the issues in https://github.com/pytorch/xla/issues/3472
Test Plan: Imported from OSS
Reviewed By: bdhirsh
Differential Revision: D35411210
Pulled By: wconstab
fbshipit-source-id: 438535e0ea2f2eb5b138e2e64b008c2213a52f94
(cherry picked from commit 6f6d2cbfb02d574147d01902d1e6b29a64150a5b)
