implement NumPy-like functionality column_stack, row_stack (#46313)
Summary:
Related https://github.com/pytorch/pytorch/issues/38349
This PR implements `column_stack` as the composite ops of `torch.reshape` and `torch.hstack`, and makes `row_stack` as the alias of `torch.vstack`.
Todo
- [x] docs
- [x] alias pattern for `row_stack`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/46313
Reviewed By: ngimel
Differential Revision: D24585471
Pulled By: mruberry
fbshipit-source-id: 62fc0ffd43d051dc3ecf386a3e9c0b89086c1d1c
