[FX] Put inf and nan in globals instead of with an import string (#47035)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/47035
Chillee thought the `from math import inf, nan` string at the top of `.code` was annoying so here's an alternative way to do it by putting those values in `globals` before we `exec`
Test Plan: Imported from OSS
Reviewed By: dzhulgakov
Differential Revision: D24611278
Pulled By: jamesr66a
fbshipit-source-id: c25ef89e649bdd3e79fe91aea945a30fa7106961
