[pytorch][codegen] move is_abstract property to NativeFunction model (#48252)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/48252
Moved to a shared place so that gen_variable_type.py can reuse it.
Test Plan: Imported from OSS
Reviewed By: ezyang
Differential Revision: D25087808
Pulled By: ljk53
fbshipit-source-id: 1f32e506956fc4eb08734cfde0add47b3e666bd9
