breakup autograd documentation (#55672)
Summary:
Related to https://github.com/pytorch/pytorch/issues/52256
Use autosummary instead of autofunction to create subpages for autograd functions. I left the autoclass parts intact but manually laid out their members.
Also the Latex formatting of the spcecial page emitted a warning (solved by adding `\begin{align}...\end{align}`) and fixed alignment of equations (by using `&=` instead of `=`).
zou3519
Pull Request resolved: https://github.com/pytorch/pytorch/pull/55672
Reviewed By: jbschlosser
Differential Revision: D27736855
Pulled By: zou3519
fbshipit-source-id: addb56f4f81c82d8537884e0ff243c1e34969a6e
