[jit] fix parser for one-line functions (#32941)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/32941
The Python grammar allows single-statement one-line functions. So we
should allow it in the string parser.
Test Plan: Imported from OSS
Differential Revision: D19704153
Pulled By: suo
fbshipit-source-id: 8c06cc9c600aa2a9567b484a1ecc0360aad443e3
