[fx] import used modules for code gen (#45471)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/45471
Intead of assuming that 'torch' is the only module used by generated code,
use the qualified names of builtin functions to generate import statements
for all builtins. This allows user-captured functions to also get code generated correctly.
Test Plan: Imported from OSS
Reviewed By: jamesr66a
Differential Revision: D23978696
Pulled By: zdevito
fbshipit-source-id: ecbff150e3de38532531cdadbfe4965468f29a38
