breakup optim, cuda documentation (#55673)
Summary:
Related to https://github.com/pytorch/pytorch/issues/52256
Use autosummary instead of autofunction to create subpages for optim and cuda functions/classes.
Also fix some minor formatting issues in optim.LBFGS and cuda.stream docstings
Pull Request resolved: https://github.com/pytorch/pytorch/pull/55673
Reviewed By: jbschlosser
Differential Revision: D27747741
Pulled By: zou3519
fbshipit-source-id: 070681f840cdf4433a44af75be3483f16e5acf7d
