[AOTI] Store OpOverload in ir.ExternKernel (#120629)
Summary: Currently the logics for filling the default value for optional arguments are scattered in several places. By storing OpOverload in the base ExternKernel class, we can simplify codegen_kwargs, and this is a preparation step for enabling the torchgen-ed C shim. The default value filling logic for FallbackKernel can also be simplified, but that can come later.
Differential Revision: [D54258089](https://our.internmc.facebook.com/intern/diff/D54258089)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/120629
Approved by: https://github.com/chenyang78
ghstack dependencies: #119987, #120592
