[NNAPI] Fix models with no weights (#47517)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/47517
While we're unlikely to see this in practice, it comes up in unit tests.
This type annotation is necessary for `torch.jit.script` to figure out
the type of the list if it is empty.
Test Plan: Unit tests in a later diff.
Reviewed By: axitkhurana
Differential Revision: D25317937
Pulled By: dreiss
fbshipit-source-id: de8b6665c6fcd3cd2b39e3c696a39336c064e4c1
