C++ tensor indexing: add Slice / TensorIndex (#30424)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30424
`at::indexing::TensorIndex` is used for converting C++ tensor indices such as `{None, "...", Ellipsis, 0, true, {1, None, 2}, torch::tensor({1, 2})}` into its equivalent `std::vector<TensorIndex>`, so that further tensor indexing operations can be performed using the supplied indices.
Test Plan: Imported from OSS
Differential Revision: D18695902
Pulled By: yf225
fbshipit-source-id: d73e14a411cdbec815866b02e75ffd71a9186e89
