[pytorch] change codegen dispatch key from string to enum (#51115)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/51115
Add enum type for dispatch key. Prepare to implement the DispatchTable
computation logic in python for static dispatch.
Verified byte-for-byte compatibility of the codegen output.
Test Plan: Imported from OSS
Reviewed By: bhosmer
Differential Revision: D26077430
Pulled By: ljk53
fbshipit-source-id: 86e74f3eb32266f31622a2ff6350b91668c8ff42
