[nnc] Add LoopNest::getLoopAt to retrieve a specified inner For-stmt (#60569)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/60569
Test Plan: Imported from OSS
Reviewed By: navahgar
Differential Revision: D29337767
Pulled By: huiguoo
fbshipit-source-id: e3ae23c1b290739c03d1fa5d7da25de878eb1d4c
