remove FunctionType as an allowed constant (#29405)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29405
We never actually used this (function attributes are a separate pathway
in ConcreteModuleType).
Test Plan: Imported from OSS
Differential Revision: D18378392
Pulled By: suo
fbshipit-source-id: b06c4b6d70f0b2534be78a215125cffd22ab44f0
