Faithful out arguments (#47712)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/47712
This adds a faithful API for ops with out arguments, as described in https://docs.google.com/document/d/1h7nBibRwkRLQ8rsPhfALlwWR0QbkdQm30u4ZBwmaps8/edit# .
After this, an op will generate the following overloads for the C++ API:
```cpp
// Generated from the aten::abs operator (NOT from aten::abs.out)
Tensor at::abs(Tensor& self)
// Generated from the aten::abs.out operator
Tensor& at::abs(Tensor& self, Tensor& out)
Tensor& at::abs_out(Tensor& out, Tensor& self)
```
This is an important step towards making those ops c10-full (it allows VariableType, XLA and other backends to ignore reordering and just call through with the same argument order), but this does not make any of those ops c10-full yet.
It enables the faithful API independent from c10-fullness. That means the API is more consistent with the same API for all ops and making an op c10-full in the future will not trigger future C++ API changes.
ghstack-source-id: 118068091
Test Plan: waitforsandcastle
Reviewed By: ezyang
Differential Revision: D24835252
fbshipit-source-id: dedfabd07140fc8347bbf16ff219aad3b20f2870
