Re-unify module and interface in ConcreteModuleType (#29825)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29825
We made `ModuleInfo` a union initially to represent the idea that a
submodule could either be a regular module or a module interface.
This PR represents module interfaces as a ConcreteModuleType with no
info (e.g. no "sugaredness"), and with the interface type as the
underlying `jitType_`. This has the effect of reducing the special
casing around adding/maintaining module info.
Test Plan: Imported from OSS
Differential Revision: D18575011
Pulled By: suo
fbshipit-source-id: 53e297b39aa1a03bcdadd795ff225aa68fec9d70
