fix bug in CompilationUnit::define (#21886)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21886
ghimport-source-id: fefbd758bbe2fbcaaad84a376ac5f69c40bccb80
Test Plan: Imported from OSS
Differential Revision: D15867647
Pulled By: suo
fbshipit-source-id: 3e0f5bbc98ec93ccf26442c4c574626e45e53888
