Remove Declarations.yaml dependency from gen_autograd (#67496)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/67496
gen_autograd.py doesn't use `Declarations.yaml` any more, and removing
the dependency allows it to run in parallel with
`tools/codegen/gen.py`.
Test Plan: Imported from OSS
Reviewed By: dagitses, ejguan
Differential Revision: D32027251
Pulled By: albanD
fbshipit-source-id: 2cc0bbe36478e6ec497f77a56ab8d01c76145703
