pytorch

d69d42db - Making ops c10 full: optional out arguments (#49083)

Making ops c10 full: optional out arguments (#49083) Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/49083 We have some (but very few) ops that take optional out arguments `Tensor(a!)? out`. This PR makes them non-optional mandatory arguments and enables c10-fullness for them. There is only a very small number of ops affected by this. Putting this up for discussion. Alternatives considered: If we keep them optional, we run into lots of issues in the dispatcher. We have to decide what the dispatcher calling convention for this argument type should be. 1) If we keep passing them in as `Tensor&` arguments and return them as `tuple<Tensor&, Tensor&, Tensor&>`, so basically same as currently, then the schema inference check will say "Your kernel function got inferred to have a `Tensor` argument but your native_functions.yaml declaration says `Tensor?`. This is a mismatch, you made an error". We could potentially disable that check, but that would open the door for real mistakes to not be reported anymore in the future. This sounds bad. 2) If we change them to a type that schema inference could differentiate from `Tensor`, say we pass them in as `const optional<Tensor>&` and return them as `tuple<const optional<Tensor>&, const optional<Tensor>&, const optional<Tensor>&>`, then our boxing logic fails because it can't recognize those as out overloads anymore and shortcut the return value as it is doing right now. We might be able to rewrite the boxing logic, but that could be difficult and could easily develop into a rabbit hole of having to clean up `Tensor&` references throughout the system where we use them. Furthermore, having optional out arguments in C++ doesn't really make sense. the C++ API puts them to the front of the argument list, so you can't omit them anyways when calling an op. You would be able to omit them when calling from Python with out kwargs, but not sure if we want that discrepancy between the c++ and python API. ghstack-source-id: 118660075 Test Plan: waitforsandcastle Reviewed By: ezyang Differential Revision: D25422197 fbshipit-source-id: 3cb25c5a3d93f9eb960d70ca014bae485be9f058