[jit] AliasDB type hash - don't always return 0 (#121874)
This hash was missing an assignment, so for almost all types it was returning "0".
c10::flat_hash_map turns out to have really bad behavior with a terrible hash like this, nearly exponential in memory usage.
Differential Revision: [D54916424](https://our.internmc.facebook.com/intern/diff/D54916424)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/121874
Approved by: https://github.com/eellison
