[FX] Delete values after their last use (#48631)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/48631
Test Plan: Imported from OSS
Reviewed By: zdevito
Differential Revision: D25235981
Pulled By: jamesr66a
fbshipit-source-id: f79d8873d3ad1ad90b5bd6367fc6119925f116e9
