bind autograd functions into C++ (#24342)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/24342
Right now the two APIs that provided in autograd package only have
python bindings and we could not call them either in C++ API or in
TorchScript. This PR make these two APIs available purely in C++ (with
preserving semantics) and can be used in C++ API and TorchScript
Differential Revision: D16923271
fbshipit-source-id: 049d6fbd94cd71ecc08b2716f74d52ac061f861e
