make gc script python2 compatible (#31536)
Summary:
get rid of f-string, somehow we still have python2
Pull Request resolved: https://github.com/pytorch/pytorch/pull/31536
Differential Revision: D19204187
Pulled By: mingbowan
fbshipit-source-id: da8e17e4dccdd6fd1b0e92eb4740f5a09a8a4209
