codegen: Split up source, header and Declarations.yaml generation (#67497)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/67497
This allows more of the code-generation to happen in parallel, whereas
previously all codegen was serialized.
Test Plan: Imported from OSS
Reviewed By: dagitses, mruberry
Differential Revision: D32027250
Pulled By: albanD
fbshipit-source-id: 6407c4c3e25ad15d542aa73da6ded6a309c8eb6a
