[doc] Fix documentations of torch functions (#52982)
Summary:
This PR includes multiple small fixes of docstrings.
* Fix documentation for [`torch.atleast_2d`](https://pytorch.org/docs/master/generated/torch.atleast_2d.html) and [`torch.atleast_3d`](https://pytorch.org/docs/master/generated/torch.atleast_3d.html) by adding a new line before `Args::`.
* Fix indentation for [`torch.isfinite`](https://pytorch.org/docs/master/generated/torch.isfinite.html) and [`torch.isinf`](https://pytorch.org/docs/master/generated/torch.isinf.html). The "Arguments", "Parameters" and "Examples" sections need to be at the same level as the first description.
* Insert a new line after `Example::` where it is missing. This makes difference in the way the documentations are rendered: see [this](https://pytorch.org/docs/master/generated/torch.gt.html) (with a new line) and [this](https://pytorch.org/docs/master/generated/torch.triu_indices.html) (without). As the majority of the docs seems to follow the former style, this PR amends the latter cases.
* Fix the "Returns" section of [`torch.block_diag`](https://pytorch.org/docs/master/generated/torch.block_diag.html) and [`torch.cartesian_prod`](https://pytorch.org/docs/master/generated/torch.cartesian_prod.html). The second and the subsequent lines shouldn't be indented, as can be seen in the docstring of [`torch.vander`](https://pytorch.org/docs/master/generated/torch.vander.html).
* Fix variable names in the example of `torch.fft.(i)fftn`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/52982
Reviewed By: mruberry
Differential Revision: D26724408
Pulled By: H-Huang
fbshipit-source-id: c65aa0621f7858b05fd16f497caacf6ea8eb33c9
