[torchgen] Add documentation for `autogen` keyword (#83610)
This is a follow up for #81437. This PR explains what operator can use `autogen` and what will be generated. Also talked about generated kernels and where to find them.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/83610
Approved by: https://github.com/albanD, https://github.com/bdhirsh
