[jit] allow compilation using optional modules (#32539)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/32539
Before: if something in `_modules` was `None`, we would barf. This is
incorrect because it's allowed for users to put `None` there, in case a
module is optional.
This case ought to be handled correctly during scripting. Fixes https://github.com/pytorch/pytorch/issues/32469
Test Plan: Imported from OSS
Differential Revision: D19552346
Pulled By: suo
fbshipit-source-id: aba7fdc19fd84d195c81cdaca8a75013a8626a8b
