Make operators with optional Tensor? arguments c10-full (#41610)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/41610
Previously, operators that have a `Tensor?` (i.e. optional tensor) in their schema implemented it using `Tensor` in C++ and filled in an undefined tensor for the None case.
The c10 operator library, however, expects `Tensor?` to be represented as `optional<Tensor>`, so those operators couldn't be c10-full yet and still had to use codegenerated unboxing instead of templated unboxing.
This PR changes that. It extends the `hacky_wrapper_for_legacy_signatures` to not only take case of TensorOptions, but now also map between signatures taking `Tensor` and `optional<Tensor>`.
For this, it requires an additional template parameter, the expected signature, and it uses that to go argument-by-argument and unwrap any optionals it finds.
ghstack-source-id: 108873701
Test Plan: waitforsandcastle
Reviewed By: bhosmer
Differential Revision: D22607879
fbshipit-source-id: 57b2fb01a294b804f82cd55cd70f0ef4a478e14f
