Update docstring on return type of `jvp` and `vjp` (#51035)
Summary:
Updates the docstrings, that `jvp` and `vjp` both return the primal `func_output` first as part of the return tuple,
in line with the docstrings of [hvp](https://github.com/niklasschmitz/pytorch/blob/c620572a3477143df33128318dc0d7d10fab811d/torch/autograd/functional.py#L671) and [vhp](https://github.com/niklasschmitz/pytorch/blob/c620572a3477143df33128318dc0d7d10fab811d/torch/autograd/functional.py#L583).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/51035
Reviewed By: bdhirsh
Differential Revision: D26047693
Pulled By: albanD
fbshipit-source-id: 5f2957a858826b4c1884590b6be7a8bed0791efd
