Enhance Tensor.unflatten to support -1 as the inferred size (#51955)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/51719, https://github.com/pytorch/pytorch/issues/28142
**Change**
- Update `torch.Tensor.unflatten` to support users pass`-1` as the inferred size for both tensors and named tensors.
- Examples of using `-1` in the `unflatten` function are added to the docs.
- Fix the rendered issue of original `unflatten` docs by removing a blank line between its example section.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/51955
Reviewed By: agolynski
Differential Revision: D26467198
Pulled By: zou3519
fbshipit-source-id: 6a3ede25561223187273796427ad0cb63f125364
