[export][ez] minor variable rename (#121040)
since `_export()` now takes an `nn.Module` only (which is asserted against at an upper layer), we should change this variable name from `f` to `mod` and remove some unnecessary isinstance checks
Differential Revision: [D54430381](https://our.internmc.facebook.com/intern/diff/D54430381/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/121040
Approved by: https://github.com/angelayi
ghstack dependencies: #121037
