Fix C++ Frontend example in frontend.html (#15717)
Summary:
The small end-to-end example in https://pytorch.org/cppdocs/frontend.html is a little outdated and needs fixes.
ezyang soumith
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15717
Differential Revision: D13591306
Pulled By: goldsborough
fbshipit-source-id: 3334d68c7f77cf094b66ec2b2f396c4c65bb0d72
