[WIP] Add support to `Tensor[]` for structured kernel codegen.
Partially fixes: #66328
This PR introduces a templated class `IList<T>`: a wrapper container for
boxed (`c10::List<T>`) and unboxed (`at::ArrayRef<T>`) containers. At this point, it was
created having `T = Tensor` in mind, but aiming for supporting `T = OptionalTensorRef`,
too.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/67964
Approved by: https://github.com/ezyang
