[C++ API] Remove init-list form of at::indexing::Slice (#34255)
Summary:
The init-list form of `at::indexing::Slice` (i.e. `tensor.index({{1, None, 2}, ...})` instead of `tensor.index({Slice(1, None, 2), ...})`) in C++ API can be easily confused with the list-form indexing in Python API (e.g. `tensor[[1, 3, 2], ...]`), which is not good from readability perspective. This PR removes the init-list form of `at::indexing::Slice` to make the API less confusing.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/34255
Test Plan: Imported from GitHub, without a `Test Plan:` line.
Differential Revision: D20290166
Pulled By: yf225
fbshipit-source-id: abbcbeca0b179219e5e1f196a33ef8aec87ebb76
