Make trivial thread-idx for degenerate statements without thread-idx. (#36480)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/36480
Test Plan: Imported from OSS
Differential Revision: D20992505
Pulled By: zheng-xq
fbshipit-source-id: 3d4e5401b59b9507b5f2db659e511bd1af53f5ab
